kernel: add a memmove() implementation for MIPS boot decompressor

This is required due to 306-mips_mem_functions_performance.patch, just
add a memmove() implementation to satisfy the lzma2 decompressor code.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43036 3c298f89-4303-0410-b956-a3cf2f4a3e73