brcm47xx: image: use standard KERNEL_IMAGE to avoid warnings

brcm47xx: image: use standard KERNEL_IMAGE to avoid warnings

KERNEL_IMAGE is used as target rule so reusing the same name causes:
Makefile:326: warning: overriding recipe for target `bin/brcm47xx/vmlinux.lzma'
Makefile:326: warning: ignoring old recipe for target `bin/brcm47xx/vmlinux.lzma'
Makefile:326: warning: overriding recipe for target `build_dir/target-mipsel_74kc+dsp2_uClibc-0.9.33.2/linux-brcm47xx_mips74k/vmlinux.lzma'
Makefile:326: warning: ignoring old recipe for target `build_dir/target-mipsel_74kc+dsp2_uClibc-0.9.33.2/linux-brcm47xx_mips74k/vmlinux.lzma'

Unfortunately this will cause copying vmlinux.lzma over and over like:
cp vmlinux.lzma FOO-kernel.bin
which is redundant on brcm47xx where we never modify kernel image.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45178 3c298f89-4303-0410-b956-a3cf2f4a3e73