payloads/external/LinuxBoot: Clean up

There were some issues with the current Linuxboot Makefiles.
- multithreaded compilation didn't work, because some prerequisites
  were missing
- initramfs wasn't added for x86 qemu boot.
- riscv support was incomplete

It began with separate patches, but resulted in a clean up patch, that
is hard to separate. The most important changes are the following:
- Instead of phony targets, actual files are now used as prerequisites
- riscv can now be used as target
- initramfs works now also for x86
- instead of querying the most recent version from the internet, I set a
  known working version (because I tested it) that can be customized
  and/or upgraded in the future. The reasons:
      - querying the version from the internet requires a constant
        connection to the internet even after linux kernel is already
        build (aka subsequent builds).
      - one usually wants to use a known working version, but optionally
        still have the posibillity to choose a custom one. This patch
        introduces this possibility in its most simple form.
- I removed as much ifeq statements as possible and moved that
  responsibility to Kconfig, because they tend to make the
  Makefile less readable.

Signed-off-by: Maximilian Brune <maximilian.brune@9elements.com>
Change-Id: I25e757108e0dd473969fe5a192ad0733f1fe6286
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76150
Reviewed-by: Lean Sheng Tan <sheng.tan@9elements.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/external/LinuxBoot/riscv/defconfig-32

@@ -0,0 +1,4 @@
+CONFIG_ARCH_RV32I=y
+CONFIG_32BIT=y
+# CONFIG_PORTABLE is not set
+CONFIG_NONPORTABLE=y

payloads/external/LinuxBoot/riscv/defconfig-64

@@ -0,0 +1,2 @@
+CONFIG_ARCH_RV64I=y
+CONFIG_64BIT=y

payloads/external/LinuxBoot/riscv/kernel_fdt_lzma.its

@@ -0,0 +1,74 @@
+/*
+ * Simple U-Boot uImage source file containing a single kernel and FDT blob
+ */
+
+/dts-v1/;
+
+/ {
+	description = "Simple image with single Linux kernel and FDT blob";
+	#address-cells = <1>;
+
+	images {
+		kernel {
+			description = "Vanilla Linux kernel";
+			data = /incbin/("../build/vmlinux.bin.lzma");
+			type = "kernel";
+			arch = "riscv64";
+			os = "linux";
+			compression = "lzma";
+			load = <0x80000>;
+			entry = <0x80000>;
+			hash-1 {
+				algo = "crc32";
+			};
+			hash-2 {
+				algo = "sha1";
+			};
+			hash-3 {
+				algo = "sha256";
+			};
+		};
+		fdt-1 {
+			description = "Flattened Device Tree blob";
+			data = /incbin/("../build/target.dtb");
+			type = "flat_dt";
+			arch = "riscv64";
+			compression = "none";
+			hash-1 {
+				algo = "crc32";
+			};
+			hash-2 {
+				algo = "sha1";
+			};
+			hash-3 {
+				algo = "sha256";
+			};
+		};
+		ramdisk-1 {
+			description = "Compressed Initramfs";
+			data = /incbin/("../build/initramfs");
+			type = "ramdisk";
+			arch = "riscv64";
+			os = "linux";
+			compression = "none";
+			load = <00000000>;
+			entry = <00000000>;
+			hash-1 {
+				algo = "sha1";
+			};
+			hash-2 {
+				algo = "sha256";
+			};
+		};
+	};
+
+	configurations {
+		default = "conf-1";
+		conf-1 {
+			description = "Boot Linux kernel with FDT blob";
+			kernel = "kernel";
+			fdt = "fdt-1";
+			ramdisk = "ramdisk-1";
+		};
+	};
+};