Makefile: set VPATH based on SRCDIR

Setting the VPATH prompts make to search VPATH for source files. There
are still some fix-ups needed for linking and including other Makefile
fragments for each architecture.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>