boot/opensbi: add support for including Linux payload

boot/opensbi: add support for including Linux payload

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
[Thomas:
 - Config.in: move the BR2_TARGET_OPENSBI_PLAT dependency to a
   "depends on", there's no need to have an if...endif block just for
   that.
 - Config.in: drop the BR2_TARGET_OPENSBI_LIBRARY_ONLY dependency,
   since this option doesn't exist
 - opensbi.mk: don't use qstrip on BR2_TARGET_OPENSBI_LINUX_PAYLOAD,
   since it's a boolean option
 - opensbi.mk: use += when adding linux to DEPENDENCIES
 - opensbki.mk: refactor the firmware file installation with a loop]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>