[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/Makefile.sbi

@@ -14,3 +14,4 @@ LDSCRIPT	= arch/riscv/scripts/sbi.lds
 # Media types
 #
 MEDIA		+= sbi
+MEDIA		+= lkrn