barebox: qstrip the path to the custom configuration file

Even though this is not strictly necessary with the current version
of barebox.mk, it becomes necessary when migrating barebox.mk to the
kconfig infrastructure.

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>