efi_loader: simplify 'printenv -e'

Currently default output of 'printenv -e' is restricted to variables with
GUID EFI_GLOBAL_VARIABLE. This excludes db and dbx. As the number of
variables is small there is no need for this restriction.

If no GUID is provided, print all matching variables irrespective of GUID.

Always show the numeric value of the GUID.

If the GUID provided to 'setenv -e' is invalid, return CMD_RET_USAGE.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>