payloads/Kconfig: Make flat-binary configurable

This adds the ability to add a flat-binary using menuconfig.

Test: boot hifive-unmatched mainboard with the following config:
CONFIG_PAYLOAD_NONE=n
CONFIG_PAYLOAD_ELF=y
CONFIG_PAYLOAD_FILE="~/repos/linux-riscv/arch/riscv/boot/Image"
CONFIG_PAYLOAD_IS_FLAT_BINARY=y
CONFIG_PAYLOAD_OPTIONS="-l 0x82000000 -e 0x82000000"
CONFIG_COMPRESSED_PAYLOAD_LZMA=y

Signed-off-by: Maximilian Brune <maximilian.brune@9elements.com>
Change-Id: I48c6b53a0c9f5b173c89f1a294a0c37fa1a58f31
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79950
Reviewed-by: Lean Sheng Tan <sheng.tan@9elements.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/payloads/Kconfig b/payloads/Kconfig
index 2a2773a..9304629 100644
--- a/payloads/Kconfig
+++ b/payloads/Kconfig
@@ -80,13 +80,14 @@
 endchoice
 
 config PAYLOAD_OPTIONS
-	string
+	string "Additional cbfstool options"
 	default ""
 	help
 	  Additional cbfstool options for the payload
 
 config PAYLOAD_IS_FLAT_BINARY
-	def_bool n
+	bool "Payload is a flat binary"
+	default n
 	help
 	  Add the payload to cbfs as a flat binary type instead of as an
 	  elf payload