ath79: move lzma-loader to the end of available RAM

ath79: move lzma-loader to the end of available RAM

In certain cases, the uncompressed initramfs image will overwrite
the lzma-loader, which is currently only 10 MB away from kernel image
start. To prevent this, change LZMA_TEXT_START to 24 MB, so loader
and compressed image have 8 MB at the end of RAM and uncompressed
image has 24 MB available.

This is only enabled for ath79 at the moment, as there we can be sure
that all devices have 32+ MB RAM and TARGET_INITRAMFS_COMPRESSION_LZMA
is not enabled there.

Despite, since lzma-loader is currently build specifically for ath79
anyway, there is no need to re-specify LOADADDR and LZMA_TEXT_START
in image/Makefile, so the values are set directly in
image/lzma-loader/Makefile and the overwrite in image/Makefile is
removed.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Tested-by: Koen Vandeputte <koen.vandeputte@ncentric.com>