efi/libstub: Stop parsing arguments at "--"

Arguments after "--" are arguments for init, not for the kernel.

Cc: <stable@vger.kernel.org>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200725155916.1376773-1-nivedita@alum.mit.edu
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>