ADS5121 DIU Add diu_bmp_addr env

Add support for using a bmp other than
FSL_Logo_BMP for the DIU splash screen.

Can now set the env var "diu_bmp_addr" to
the address of a BMP in flash to use instead
of the default FSL_Logo_BMP.

Signed-off-by: Martha Marx <mmarx@silicontkx.com>
Signed-off-by: John Rigby <jrigby@freescale.com>