ar71xx: fix board detection with newer RouterBOOT versions

ar71xx: fix board detection with newer RouterBOOT versions

Recent RouterBOOT version (at least version 3.41 on RB911G-5HPacD)
use "Board=" kernel parameter instead of "board=" to pass the board
name to the kernel. Due to this change the board detection code is
not working on the devices shipped with the new RouterBOOT version.
Because the kernel is unable to identify these boards they become
unusable despite that they are supported by the current code.

Update the prom_init code to convert the 'Board' kernel parameter to
'board'. After this change, the board detection works also with the
new RouterBOOT versions.

Signed-off-by: Gabor Juhos <juhosg@freemail.hu>