AndroidKernel.mk: Add support for multi kernel tree

We now support building Android in a build tree that contains
multiple kernels. The kernel version to be built is passed in
by the higher level makefiles.

Currently supported structure:
- legacy <Android Root>/kernel
- New <Android Root>/kernel/msm-*

Change-Id: Ibd6c03c019643adfbadc61e46a3dd760930028bb
Signed-off-by: Ameya Thakur <ameyat@codeaurora.org>
Signed-off-by: David Ng <dave@codeaurora.org>