boot/barebox: check for config file before calling kconfig-package

If we check that the user provides a config file after we call to the
kconfig-package infra, the error message we get is the one for the
kconfig-package infra, not the custom error message we want to show to
the user.

So, only call kconfig-package after we do the check.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

boot/barebox/barebox.mk

@@ -109,10 +109,12 @@ define BAREBOX_INSTALL_TARGET_CMDS
 endef
 endif
 
-$(eval $(kconfig-package))
-
+# Checks to give errors that the user can understand
+# Must be before we call to kconfig-package
 ifeq ($(BR2_TARGET_BAREBOX)$(BR_BUILDING),yy)
 ifeq ($(BAREBOX_SOURCE_CONFIG),)
 $(error No Barebox config file. Check your BR2_TARGET_BAREBOX_BOARD_DEFCONFIG or BR2_TARGET_BAREBOX_CUSTOM_CONFIG_FILE settings)
 endif
 endif
+
+$(eval $(kconfig-package))