linux-at91: Add support to select source-tree defconfig
linux-at91: Add support to select source-tree defconfig

At this moment, all our machines have a corresponding defconfig, used to
build the kernel. However, some machines don't necessarily need a different
defconfig than the one present in the source-tree. This patch adds support
to select a defconfig from source-tree by setting it in KBUILD_DEFCONFIG
variable. In case KBUILD_DEFCONFIG is set and defconfig file is present,
the defconfig file takes priority and KBUILD_DEFCONFIG is ignored.

Signed-off-by: Codrin Ciubotariu <97034fe81dd68ee5e1fa3aa6216f6373b034daef@microchip.com>

linux4sam/meta-atmel,linux4sam/meta-atmel,linux4sam/meta-atmel,linux4sam/meta-atmel,linux4sam/meta-atmel