efibootmgr: Check for efi supported after parsing args

Move the check for efi variables being supported to after parsing the args. This
allows '-h' to produce both as a normal user as well as on all systems.

Approved by: re@ (gjb)

(cherry picked from commit 7fe2f504f8a0e4237872f8528e911c5f7b7ed59d)
(cherry picked from commit b8b867a10f56b4a78b680fb5d2a97c83e745a0ce)