configs/qemu_xtensa_lx60_*: kernel build needs mkimage

configs/qemu_xtensa_lx60_*: kernel build needs mkimage

Following ffbe46a5295 ("linux: simplify LINUX_BUILD_CMDS"), the Linux
kernel build for these xtensa qemu builds an image format that needs
mkimage.

Reported-by: Jugurtha BELKALEM <jugurtha.belkalem@smile.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Jugurtha BELKALEM <jugurtha.belkalem@smile.fr>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>