Enable mixed build for GKI-based targets. Mixed build works by first
compiling the QKI kernel source tree to generate vmlinux, Image, and
other artifacts. Then, msm-kalama kernel tree modules are compiled.
The artifacts generated by these compilations are combined
to produce the final set of boot, vendor_boot images, etc.

Change-Id: I768e3df9297362aae04b980db41aeb52178b27f6
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>