efi_loader: don't use unlimited stack as buffer

The length of a string printed to the console by the
EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL is not limited by the UEFI spec.
Hence should not allocate a buffer for it on the stack.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>