Change max size of uncompressed uImage's to 8MByte and add

CFG_BOOTM_LEN to adjust this setting.

As mentioned by Robin Getz on 2005-05-24 the size of uncompressed
uImages was restricted to 4MBytes. This default size is now
increased to 8Mbytes and can be overrided by setting CFG_BOOTM_LEN
in the board config file.

Patch by Stefan Roese, 13 Mar 2006