Adding support for saving build objects in a separated directory.

Adding support for saving build objects in a separated directory.

This patch basically ports the same functionality already available at u-boot,
and can be used in two ways:
 1) Add O= to the make command line
    'make O=/tmp/build board_config'
    'make O=/tmp/build ift'

 2) Set environement variable BUILD_DIR to point to the desired location
    'export BUILD_DIR=/tmp/build'
    'make board_config'
    'make ift'

Similar to u-boot, setting 'O=' at the command line will override the BUILD_DIR
environment variable.

If you use none of above methods, the build will happen localy, as before.

Changes since v1:
 * Rebase to correctly apply against the main Makefile after the signGP merge

Signed-off-by: Ricardo Salveti de Araujo <ricardo.salveti@canonical.com>
Acked-by: Nishanth Menon <nm@ti.com>