linux: fix fallout from c78d57637cd57a383e44713eb27e654b3071340e

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>

linux/linux.mk

@@ -106,7 +106,9 @@ LINUX_IMAGE_NAME = vmlinux
 else ifeq ($(BR2_LINUX_KERNEL_VMLINUZ),y)
 LINUX_IMAGE_NAME = vmlinuz
 endif
+LINUX_TARGET_NAME = $(LINUX_IMAGE_NAME)
 endif
+
 ifeq ($(LINUX_IMAGE_NAME),)
 LINUX_IMAGE_NAME = $(LINUX_TARGET_NAME)
 endif