Don't compile qemu when gem5 is used

build

@@ -94,6 +94,9 @@ fi
 if $x11; then
   cat ../buildroot_config_fragment_x11 >> "${outdir}/.config"
 fi
+if ! "$gem5"; then
+  cat ../buildroot_config_fragment_qemu >> "${outdir}/.config"
+fi
 if [ "$arch" = 'mips64' ]; then
   # Workaround for:
   # http://lists.busybox.net/pipermail/buildroot/2017-August/201053.html

buildroot_config_fragment

@@ -27,14 +27,6 @@ BR2_OPTIMIZE_0=y
 BR2_PACKAGE_GDB=y
 BR2_PTHREAD_DEBUG=y
 
-# QEMU
-BR2_PACKAGE_HOST_QEMU=y
-# False because otherwise we need the host to be as recent as guest, and the build fails with:
-# package/qemu/qemu.mk:110: *** "Refusing to build qemu-user: target Linux version newer than host's.".  Stop.
-BR2_PACKAGE_HOST_QEMU_LINUX_USER_MODE=n
-BR2_PACKAGE_HOST_QEMU_SYSTEM_MODE=y
-BR2_PACKAGE_HOST_QEMU_VDE2=y
-
 # ftrace
 BR2_PACKAGE_TRACE_CMD=y
 

buildroot_config_fragment_qemu

@@ -0,0 +1,6 @@
+BR2_PACKAGE_HOST_QEMU=y
+# False because otherwise we need the host to be as recent as guest, and the build fails with:
+# package/qemu/qemu.mk:110: *** "Refusing to build qemu-user: target Linux version newer than host's.".  Stop.
+BR2_PACKAGE_HOST_QEMU_LINUX_USER_MODE=n
+BR2_PACKAGE_HOST_QEMU_SYSTEM_MODE=y
+BR2_PACKAGE_HOST_QEMU_VDE2=y

rootfs_post_image_script

@@ -2,6 +2,7 @@
 images_dir="$1"
 cd "$images_dir"
 f=rootfs.ext2
-if [ -f "$f" ]; then
-  "${HOST_DIR}/bin/qemu-img" convert -f raw -O qcow2 "$f" "${f}.qcow2"
+exe="${HOST_DIR}/bin/qemu-img"
+if [ -f "$exe" ] && [ -f "$f" ]; then
+   "$exe" convert -f raw -O qcow2 "$f" "${f}.qcow2"
 fi