imx: add writebcb command

imx: add writebcb command

Add writebcb command which creates a NAND Boot Configuration Block
(BCB) at the beginning of the active flash device. The BCB stores
the information for the SoC internal boot ROM where the application
with a valid IVT header can be found on the NAND device. The first
two argument of the command need an offset of the NAND device where
the primary and secondary application can be found.

Typically, U-Boot is the application which gets loaded by the boot
ROM. Hence, the offset address need to be the address where U-Boot
(u-boot.imx along with a 0x400 long prefix) is stored on the device.
At least one location is mandatory.

Currently only the FCB (Firmware Configuration Block) is written to
the device. The DBBT (Discovered Bad Block Table) is optional and
not created by writebcb currently.