linux: fix breakage from d4b2b032

Unbreak qemu_xtensa_lx60_defconfig where LINUX_IMAGE_NAME !=
LINUX_TARGET_NAME.
It incorrectly overwrites LINUX_IMAGE_NAME even if it was set before,
defeating the purpose of IMAGE being different than TARGET.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>