opensbi-payloads.inc: Use strings for fallback

This function is evaluated during parse and some machines maynot use
external payload, on such machines this fails to parse

Fixes
bb.data_smart.ExpansionError: Failure expanding variable do_compile[depends], expression was  ${@riscv_get_do_compile_depends(d)} which triggered exception TypeError: argument of type 'NoneType' is not iterab
le

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>