ANDROID: Creating boot.img for x86_64 GKI

The boot.img will be used for GKI testing.
Also removing BUILD_GKI_CERTIFICATION_TOOLS=1, because
we only need to certify GKI boot-*.img for aarch64.

Bug: 232906147
Test: BUILD_CONFIG=common/build.config.gki.x86_64 build/build.sh
Signed-off-by: Bowgo Tsai <bowgotsai@google.com>
Change-Id: Ia6790dc9faddce7c616411d7ec5c1f60a12aea44
(cherry picked from commit a80c9ffa)