[lkrn] Add basic support for the RISC-V Linux kernel image format

The RISC-V and AArch64 bare-metal kernel images share a common header
format, and require essentially the same execution environment: loaded
close to the start of RAM, entered with paging disabled, and passed a
pointer to a flattened device tree that describes the hardware and any
boot arguments.

Implement basic support for executing bare-metal RISC-V and AArch64
kernel images.  The (trivial) AArch64-specific code path is untested
since we do not yet have the ability to build for any bare-metal
AArch64 platforms.  Constructing and passing an initramfs image is not
yet supported.

Rename the IMAGE_BZIMAGE build configuration option to IMAGE_LKRN,
since "bzImage" is specific to x86.  To retain backwards compatibility
with existing local build configurations, we leave IMAGE_BZIMAGE as
the enabled option in config/default/pcbios.h and treat IMAGE_LKRN as
a synonym for IMAGE_BZIMAGE when building for x86 BIOS.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/bits/lkrn.h

@@ -0,0 +1,17 @@
+#ifndef _BITS_LKRN_H
+#define _BITS_LKRN_H
+
+/** @file
+ *
+ * Dummy architecture-specific Linux kernel image invocation
+ *
+ * This file is included only if the architecture does not provide its
+ * own version of this file.
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#define LKRN_MAGIC_ARCH 0
+
+#endif /* _BITS_LKRN_H */