efi_loader: factor out efi_set_load_options()

The bootefi bootmgr command has to set the load options for a loaded image
from the value of BootXXXX variable. If the boot manager is not used, the
value is set from the environment variable bootargs (or efi_selftest).

Factor out a common function efi_set_load_options().

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