[riscv] Add a .lkrn build target resembling a Linux kernel binary

On x86 BIOS, it has been useful to be able to build iPXE to resemble a
Linux kernel, so that it can be loaded by programs such as syslinux
which already know how to handle Linux kernel binaries.

Add an equivalent .lkrn build target for RISC-V SBI, allowing for
build targets such as:

  make bin-riscv64/ipxe.lkrn

  make bin-riscv64/cgem.lkrn

The Linux kernel header format allows us to specify a required length
(including uninitialised-data portions) and defines that the image
will be loaded at a fixed offset from the start of RAM.  We can
therefore use known-safe areas of memory (within our own .bss) for the
initial temporary page table and stack.

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

src/arch/riscv/prefix/libprefix.S

@@ -452,6 +452,7 @@ paging_mode_names:
 	 * This can be used only once .bss is known to be writable.
 	 */
 	.section ".bss.page_table", "a", @nobits
+	.globl	page_table
 	.balign	PAGE_SIZE
 page_table:
 	.space	PAGE_SIZE