ANDROID: fix KCFLAGS override by __ANDROID_COMMON_KERNEL__

Our test build is broken by KCFLAGS overrided in build.config.comm.

Since Linux Makefile supports 'export KCFLAGS=XXX' to customize the
KCFLAGS, and we should keep this functionality.

Bug: 230818006
Fixes: 5558db26 ("ANDROID: Add flag to indicate compiling against ACK")
Signed-off-by: Lecopzer Chen <lecopzer.chen@mediatek.com>
Change-Id: I9425d79697bc1fe816ce82d523f91631dee6b8f4