Fixes 2 problems related to multi-kernel tree support:

1. Copying of modules to /system/lib/modules is broken when building
in a multi-kernel tree. This is because INSTALL_MOD_PATH is not
set correctly. When building a multi-kernel tree, the output
directory is one additional directory deep, so modules end up
under <out>/obj/system/lib/modules instead of
<out>/system/lib/modules. Fix this by using BUILD_ROOT_LOC
which is set appropriately for multi-kernel and standard trees.
2. When running "make kernelconfig" on a multi-kernel tree,
the generated defconfig is copied to the wrong location,
since it uses the old-style location under kernel, instead
of kernel/<kernel name>.

Change-Id: I90563104a5b6219472eaeae1964fc34b52586536
CRs-Fixed: 1014872
Signed-off-by: Lior David <liord@codeaurora.org>