linux: qstrip the path to the custom configuration file

Even though this is not strictly necessary with the current version of
linux.mk, it becomes necessary when migrating linux.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>