Makefile: Allow for direct build in makefile wrapper

Currently, if you are building with the makefile wrapper, the build
directory that is specified gets modified. There are cases when this
is unwanted, this patch adds a DIRECT_BUILD variable so we can build
directly to a specific directory without having extra fields appended
to our build directory.

Signed-off-by: Haaris Farooq <haaris.farooq@arm.com>