Move arm to virt armv7, unify arm QEMU and gem5 builds

gem5 requires armv7, and we learnt that the versatiledb we were using
was pre-v7.

We could have moved to -M vexpress-*, but in the end decided to go for
-M virt due to its simpliciy, and uniformity with aarch64.

platform_device: does not work anymore and was removed, since it was tied
to versatilepb.

We left a mention on the README and removed all in tree source. The QEMU
patch is still left as it was.

As a consequence, the linux tree had no other patches, and we now use
vanilla linux by default, which is a great thing for reproducibility.

Another consequence is the /poweroff.out works for arm -M virt,
and we removed all mentions of the problem.

gem5/external.mk

@@ -17,6 +17,8 @@ endif
 define GEM5_BUILD_CMDS
 	# Cannot pass "-c '$(TARGET_CROSS)'" here because the ARM build uses aarch64 for the bootloader...
 	cd '$(@D)' && ./build -a '$(ARCH)' -j '$(BR2_JLEVEL)'
+	# TODO cannot use TARGET_CONFIGURE_OPTS here because it overrides the CFLAGS on m5,
+	# which have an include. We should patch gem5 to add a += instead of = there.
 	cd '$(@D)/gem5/util/m5' && $(MAKE) -f 'Makefile.$(ARCH_MAKE)' CC='$(TARGET_CC)' LD='$(TARGET_LD)'
 endef