ANDROID: GKI: Add support for a GKI_DEFCONFIG_FRAGMENT

To provide more fleixbility in debugging GKI builds, create a mechanism
for adding an optional file which may override some of the build vars,
such as POST_DEFCONFIG_CMDS.

Bug: 171343315
Signed-off-by: J. Avila <elavila@google.com>
Change-Id: I9f4a7bad2c19bcdb3b494c97d7c1eea5c7f311ba

build.config.gki.aarch64

@@ -25,3 +25,7 @@ TRIM_NONLISTED_KMI=1
 KMI_SYMBOL_LIST_STRICT_MODE=${KMI_SYMBOL_LIST_STRICT_MODE:-1}
 MODULES_ORDER=android/gki_aarch64_modules
 KMI_ENFORCED=1
+
+if [ -n "${GKI_DEFCONFIG_FRAGMENT}" ]; then
+source ${GKI_DEFCONFIG_FRAGMENT}
+fi