linux: fix fallout from c78d5763

The new variable LINUX_TARGET_NAME is unconditionally used but it may be
unset leading to a default kernel build (which might not be uImage or
other requested format).
See http://lists.busybox.net/pipermail/buildroot/2014-July/102069.html



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