HV: make: manage debug/release build in kconfig

This patch introduces a configuration symbol RELEASE for managing debug/release
build in a similar way to how we manage PLATFORM.

Note:

1. 'make defconfig RELEASE=1' will still use the CONFIG_RELEASE defined in the
   default configuration. The 'RELEASE=1' option has no effect in this case.

2. 'make RELEASE=1' is backward-compatible and enforces a release version to be
   built.

v1 -> v2:

    * Pass RELEASE instead of CONFIG_RELEASE to silentoldconfig.py to avoid
      unintended overriding of the value.

Signed-off-by: Junjie Mao <junjie.mao@intel.com>
Acked-by: Anthony Xu <anthony.xu@intel.com>
Acked-by: Geoffroy VanCutsem <geoffroy.vancutsem@intel.com>

hypervisor/arch/x86/Kconfig

@@ -16,6 +16,10 @@ config PLATFORM
 	default "uefi" if PLATFORM_UEFI
 	default "sbl" if PLATFORM_SBL
 
+config RELEASE
+	bool "Release build"
+	default n
+
 config NR_IOAPICS
 	int "Maximum number of IOAPICs supported"
 	default 1