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.

build

@@ -97,7 +97,7 @@ case "$arch" in
     defconfig=qemu_x86_64_defconfig
   ;;
   arm)
-    defconfig=qemu_arm_versatile_defconfig
+    defconfig=qemu_arm_vexpress_defconfig
   ;;
   aarch64)
     defconfig=qemu_aarch64_virt_defconfig
@@ -130,7 +130,7 @@ BR2_ROOTFS_POST_SCRIPT_ARGS=\"${post_script_args}\"
 " >> "$config_file"
   if "$gem5"; then
     printf "BR2_PACKAGE_GEM5=y\n" >> "${config_file}"
-    if [ -z "$linux_kernel_custom_config_file" ] && [ ! "$arch" = aarch64 ]; then
+    if [ -z "$linux_kernel_custom_config_file" ] && [ "$arch" = x86_64 ]; then
       f="../kernel_config_${arch_dir}"
       if [ -f "$f" ]; then
         printf "BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE=\"$f\"\n" >> "$config_file"