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>

linux/linux.mk

@@ -171,7 +171,7 @@ LINUX_POST_PATCH_HOOKS += LINUX_APPLY_PATCHES
 ifeq ($(BR2_LINUX_KERNEL_USE_DEFCONFIG),y)
 KERNEL_SOURCE_CONFIG = $(KERNEL_ARCH_PATH)/configs/$(call qstrip,$(BR2_LINUX_KERNEL_DEFCONFIG))_defconfig
 else ifeq ($(BR2_LINUX_KERNEL_USE_CUSTOM_CONFIG),y)
-KERNEL_SOURCE_CONFIG = $(BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE)
+KERNEL_SOURCE_CONFIG = $(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE))
 endif
 
 define LINUX_CONFIGURE_CMDS