efi: Add efitextmode command for getting/setting the text mode resolution

efi: Add efitextmode command for getting/setting the text mode resolution

This command is meant to behave similarly to the "mode" command of the EFI
Shell application. In addition to allowing mode selection by giving the
number of columns and rows as arguments, the command allows specifying the
mode number to select the mode. Also supported are the arguments "min" and
"max", which set the mode to the minimum and maximum mode respectively as
calculated by the columns * rows of that mode.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>