linux: adjust BR2_LINUX_KERNEL_IMAGE_NAME help text

linux: adjust BR2_LINUX_KERNEL_IMAGE_NAME help text

The help text of BR2_LINUX_KERNEL_IMAGE_NAME is somewhat incomplete,
in the sense that it assumes just a filename can be passed, while it
can be a relative path, such as 'compressed/vmlinux.bin.z'. So make it
clear that such paths are relative to arch/ARCH/boot/.

Also, drop the part about this being only useful for Xtensa as this is
not true: on MIPS it might be needed as well for some specific image
types.

Reported-by: Paul Cercueil <paul@crapouillou.net>
Cc: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>