cmd: efidebug: add drivers command

"drivers" command prints all the uefi drivers on the system.

=> efi drivers
Driver           Name                 Image Path
================ ==================== ====================
000000007ef003d0 <NULL>               <built-in>

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