kconfig: support board-specific defconfig

The current defconfigs are BIOS-specific which makes it difficult to maintain
multiple defconfigs for boards running the same BIOS.

This patch re-organizes the defconfigs to be board-specific. A command line
option BOARD is introduced to specify a board on which the current build targets
at. The original PLATFORM is kept for backward compatibility which redirects to
apl-mrb and nuc6cayh for sbl and uefi, respectively.

The getting started guide is also updated accordingly.

v1 -> v2:

* Rewrite 'up2' to 'UP2'.

Tracked-On: #1588
Signed-off-by: Junjie Mao <junjie.mao@intel.com>
Reviewed-by: Anthony Xu <anthony.xu@intel.com>

hypervisor/arch/x86/Kconfig

@@ -45,6 +45,16 @@ config PLATFORM
 	default "uefi" if PLATFORM_UEFI
 	default "sbl" if PLATFORM_SBL
 
+config BOARD
+	string "Target board"
+	help
+	The target board this build runs on top of.
+
+config DEFCONFIG_LIST
+	string
+	option defconfig_list
+	default "arch/x86/configs/$BOARD.config"
+
 config RELEASE
 	bool "Release build"
 	default n

hypervisor/arch/x86/configs/apl-mrb.config

@@ -1,2 +1,3 @@
+CONFIG_BOARD="apl-mrb"
 # CONFIG_PLATFORM_UEFI is not set
 CONFIG_PLATFORM_SBL=y

hypervisor/arch/x86/configs/apl-nuc.config

@@ -0,0 +1 @@
+nuc6cayh.config

hypervisor/arch/x86/configs/nuc6cayh.config

@@ -0,0 +1,3 @@
+CONFIG_BOARD="NUC6CAYH"
+CONFIG_PLATFORM_UEFI=y
+# CONFIG_PLATFORM_SBL is not set

hypervisor/arch/x86/configs/up2.config

@@ -1,2 +1,3 @@
+CONFIG_BOARD="UP2"
 CONFIG_PLATFORM_UEFI=y
 # CONFIG_PLATFORM_SBL is not set