brcm63xx: add working lzma-loader and use it for initramfs

Add a working lzma loader and use it for generating initramfs kernels
to allow easily netbooting elf kernels on devices with a 4 MiB CFE
size limit.

Based on ar71xx's lzma-loader.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41940 3c298f89-4303-0410-b956-a3cf2f4a3e73