tools: vmcfg: use defconfig instead of default values in Kconfig

The current vmcfg uses the default values in Kconfig when a previous .config
does not exist. This leads to additional complexity to silentoldconfig.py which
has different logic depending on the environment variable 'BOARD'. This also
blocks the effort to make the top-level Makefile recognize BOARD because any
environment variable set in the top-level Makefile cascades to the other
Makefiles, leading the work around above to fail.

This patch introduces a generic defconfig for vmcfg and simplifies
silentoldconfig.py to always fail when neither .config nor the specified
defconfig exists.

Tracked-On: #1995
Signed-off-by: Junjie Mao <junjie.mao@intel.com>
Signed-off-by: Tw <wei.tan@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@intel.com>
Reviewed-by: Yin Fengwei <fengwei.yin@intel.com>

devicemodel/vmcfg/apl-mrb/vm1/Kconfig

@@ -1,3 +1,8 @@
+config DEFCONFIG_LIST
+	string
+	option defconfig_list
+	default "apl-mrb/vm1/configs/default.config"
+
 menuconfig MRB_VM1
 	bool "Build-in VM1 configuration for MRB"
 	default n

devicemodel/vmcfg/apl-mrb/vm1/configs/default.config

@@ -0,0 +1,2 @@
+# Generated by Kconfiglib (https://github.com/ulfalizer/Kconfiglib)
+# CONFIG_VM_CFG is not set

scripts/kconfig/silentoldconfig.py

@@ -52,42 +52,39 @@ def main():
     # the defconfig will be used as the base config. Otherwise the existing
     # .config is used as the base.
     #
-    # Note that the BOARD given by the environment variable may be different
-    # from what is specified in the corresponding defconfig. Thus we get the
-    # target_board by loading the defconfig first.
-    #
-    # To be backward compatible, do not look for any defconfig in the Kconfig if
-    # such BOARD is not given. Simply use the .config or all default values
-    # (when no .config exists) as the base.
+    # If .config does not exist, it is required that Kconfig specifies an
+    # existing defconfig, otherwise this script will refuse to generate a
+    # .config.
     config_path = sys.argv[2]
-    target_board = None
-    if 'BOARD' in os.environ:
     defconfig_path = kconfig.defconfig_filename
     if defconfig_path and os.path.isfile(defconfig_path):
-            kconfig.load_config(defconfig_path)
-            target_board = kconfig.syms['BOARD'].str_value
+        kdefconfig = kconfiglib.Kconfig(kconfig_path)
+        kdefconfig.load_config(defconfig_path)
+    else:
+        kdefconfig = None
 
     need_update = False
     if os.path.isfile(config_path):
         kconfig.load_config(config_path)
-        if target_board and kconfig.syms['BOARD'].str_value != target_board:
-            kconfig.load_config(defconfig_path)
+        # The BOARD given by the environment variable may be different from what
+        # is specified in the corresponding defconfig. So compare the value of
+        # CONFIG_BOARD directly. This is applicable only when CONFIG_BOARD
+        # exists in the Kconfig.
+        if kdefconfig and 'BOARD' in kconfig.syms and \
+           kconfig.syms['BOARD'].str_value != kdefconfig.syms['BOARD'].str_value:
+            kconfig = kdefconfig
             sys.stdout.write("Overwrite with default configuration based on %s.\n" % defconfig_path)
             need_update = True
     else:
-        # create a default configuration
-        if target_board:
-            # target_board is not None if and only if a BOARD is specified and the
-            # corresponding defconfig has been loaded.
+        # base on a default configuration
+        if kdefconfig:
+            kconfig = kdefconfig
             sys.stdout.write("Default configuration based on %s.\n" % defconfig_path)
-        else:
-            # If no known defconfig exists, either report an error if a
-            # BOARD is specified in the environment, or use the default values in
-            # Kconfig.
-            if 'BOARD' in os.environ:
-                sys.stderr.write("No defconfig found for board %s.\n" % target_board)
-                sys.exit(1)
             need_update = True
+        else:
+            # report an error if no known defconfig exists
+            sys.stderr.write(".config does not exist and no defconfig available.\n")
+            sys.exit(1)
 
     # Update the old .config with those specified on cmdline
     #