tools: imximage: add set bit command

Add set bit command support.
Usage: SET_BIT 4 [address] [bitmask]

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Stefano Babic <sbabic@denx.de>