ANDROID: refactor build.config files to remove duplication

The build.config.* files largely contain duplicate information by their
nature. Reorganize them reduce duplication and to allow adding new
configurations without copying the definitions again.

Bug: 140224784
Change-Id: I6a3810a125b0ed48591690ca33bb5c02be58218a
Signed-off-by: Matthias Maennich <maennich@google.com>
diff --git a/build.config.gki b/build.config.gki
new file mode 100644
index 0000000..66f6eb8
--- /dev/null
+++ b/build.config.gki
@@ -0,0 +1,4 @@
+DEFCONFIG=gki_defconfig
+POST_DEFCONFIG_CMDS="check_defconfig"
+BUILD_INITRAMFS=1
+