Support MFG.
1. When ROM jumps to SPL, SPL saves ROM context. After SPL successfully
   initialize DRAM, SPL will restore ROM and context and back to ROM.
   During this flow, SPL does not do any USB configuration to avoid
   breaking the connect between ROM and Host PC, because we rely on
   ROM to continue serial download with Host.
2. Add MFG env to support kernel boot correctly.

Signed-off-by: Peng Fan <peng.fan@nxp.com>