roms: add microvm-bios (qboot) as binary and git submodule

qboot is a minimalist x86 firmware for booting Linux kernels. It does
the mininum amount of work required for the task, and it's able to
boot both PVH images and bzImages without relying on option roms.

This characteristics make it an ideal companion for the microvm
machine type.

Signed-off-by: Sergio Lopez <slp@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>

.gitmodules

@@ -58,3 +58,6 @@
 [submodule "roms/opensbi"]
 	path = roms/opensbi
 	url = 	https://git.qemu.org/git/opensbi.git
+[submodule "roms/qboot"]
+	path = roms/qboot
+	url = https://github.com/bonzini/qboot

roms/Makefile

@@ -67,6 +67,7 @@ default help:
 	@echo "  opensbi32-virt     -- update OpenSBI for 32-bit virt machine"
 	@echo "  opensbi64-virt     -- update OpenSBI for 64-bit virt machine"
 	@echo "  opensbi64-sifive_u -- update OpenSBI for 64-bit sifive_u machine"
+	@echo "  bios-microvm       -- update bios-microvm.bin (qboot)"
 	@echo "  clean              -- delete the files generated by the previous" \
 	                              "build targets"
 
@@ -186,6 +187,10 @@ opensbi64-sifive_u:
 		PLATFORM="sifive/fu540"
 	cp opensbi/build/platform/sifive/fu540/firmware/fw_jump.bin ../pc-bios/opensbi-riscv64-sifive_u-fw_jump.bin
 
+bios-microvm:
+	$(MAKE) -C qboot
+	cp qboot/bios.bin ../pc-bios/bios-microvm.bin
+
 clean:
 	rm -rf seabios/.config seabios/out seabios/builds
 	$(MAKE) -C sgabios clean
@@ -198,3 +203,4 @@ clean:
 	$(MAKE) -C skiboot clean
 	$(MAKE) -f Makefile.edk2 clean
 	$(MAKE) -C opensbi clean
+	$(MAKE) -C qboot clean