barebox: migrate to the kconfig infrastructure

[Thomas:
 - use $(BAREBOX_DIR) instead of $(@D) when defining
   BAREBOX_SOURCE_CONFIG, because $(@D) has no value at this point
   since we're outside of a make rule. This was causing Barebox to be
   constantly rebuilt, since the defconfig path was not a full path,
   it was looking like: '/arch/arm/configs/tegra_v7_defconfig'. The
   solution of using $(BAREBOX_DIR) has been used to mimic was is done
   in the linux package, which uses $(LINUX_DIR).]

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: "Yann E. Morin" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>