buildroot

linux/linux.mk: use namespace for internal variables

These three symbols:
KERNEL_ARCH_PATH
KERNEL_DTBS
KERNEL_DTS_NAME
are defined and used only inside this file, so use the LINUX_ namespace
for them instead of KERNEL_.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>