cmd: gpt: Add option to write GPT partitions to environment variable

This change would enhance the existing 'gpt read' command to allow
(optionally) writing of the read GPT partitions to an environment
variable in the UBOOT partitions layout format. This would allow users
to easily change the overall partition settings by editing said variable
and then using the variable in the 'gpt write' and 'gpt verify' commands.

Signed-off-by: Farhan Ali <farhan.ali@broadcom.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Corneliu Doban <cdoban@broadcom.com>
Cc: Rayagonda Kokatanur <rayagonda.kokatanur@broadcom.com>
Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>