Makefile: add empty Makefile.boot file

Because 32bit target disable ARCH_MULTIPLATFORM, so when compile
kernel, compilor always want to find Makefile.boot file, there is
some compile issue if not find this file, so add Makefile.boot
file.

Change-Id: I17be090d180502f72f18135fbb55446b325c07d0
Signed-off-by: Tengfei Fan <tengfeif@codeaurora.org>
Signed-off-by: Cong Zhang <congzhan@codeaurora.org>