igep00x0: merge igep0020 and igep0030 defconfigs to igep00x0_defconfig

igep00x0: merge igep0020 and igep0030 defconfigs to igep00x0_defconfig

Update igep00x0 code with the following features:
- Add board and revision detection for the boards:
  - IGEP0020-RF
  - IGEP0020-RC
  - IGEP0030-RG
  - IGEP0030-RE
- Merge IGEP0020 and IGEP0030 mux tables
- Add suport to use GPIO_126, GPIO_127 and GPIO_129
- board_name and board_rev environment variables display board and
  revision informations
- Move dtb name selection from code to boot script

Signed-off-by: Pau Pajuelo <ppajuel@gmail.com>
Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Tested-by: Pau Pajuelo <ppajuel@gmail.com>