cmd: env: add "-e" option for handling UEFI variables

"env [print|set] -e" allows for handling uefi variables without
knowing details about mapping to corresponding u-boot variables.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>