cmd: add efidebug command

Currently, there is no easy way to add or modify UEFI variables.
In particular, bootmgr supports BootOrder/BootXXXX variables, it is
quite hard to define them as u-boot variables because they are represented
in a complicated and encoded format.

The new command, efidebug, helps address these issues and give us
more friendly interfaces:
 * efidebug boot add: add BootXXXX variable
 * efidebug boot rm: remove BootXXXX variable
 * efidebug boot dump: display all BootXXXX variables
 * efidebug boot next: set BootNext variable
 * efidebug boot order: set/display a boot order (BootOrder)

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