ARM: Fix build error when using KCONFIG_CONFIG

Jonathan Cameron reports that when using the environment
variable KCONFIG_CONFIG, he encounters this error:

make[2]: *** No rule to make target `.config', needed by `arch/arm/boot/compressed/vmlinux.lds'

Reported-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>