board: freescale: p1_p2_rdb_pc: Add env commands norlowerboot, norupperboot, sd2boot and defboot

All *boot env commands overrides default boot source location via i2c.
After board reset without power off, BootROM then starts booting U-Boot
from this specified location instead of the default one.

Add new env command defboot which reverts boot location to the default
value, which in most cases is configurable by HW DIP switches.

And add new env commands norlowerboot, norupperboot, sd2boot to boot from
other locations. norlowerboot would instruct BootROM to boot from lower NOR
bank, norupperboot from upper NOR bank and sd2boot from SD card with
alternative configuration.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>