cmd: gpt: add eMMC and GPT support

Add eMMC and GPT support.
- GPT partition list and command to create the GPT added to u-boot
  environment
- eMMC boot commands added to u-boot environment
- new gpt commands (enumarate and setenv) that are used by broadcom
  update scripts and boot commands
- eMMC specific u-boot configurations with environment saved in eMMC
  and GPT support

Signed-off-by: Corneliu Doban <cdoban@broadcom.com>
Signed-off-by: Rayagonda Kokatanur <rayagonda.kokatanur@broadcom.com>
[trini: Make gpt_setenv initialize ret to -1, given relevant return
        error values]
Signed-off-by: Tom Rini <trini@konsulko.com>