runqemu: support rootfs mounted ro

Optionally allow to set QB_KERNEL_ROOT to e.g. "/dev/vda ro" to mount the
rootfs reay-only in Qemu.

(From OE-Core rev: 448eb1fd21287ba16b17e9402ce040b86ae3638c)

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/runqemu

@@ -1266,7 +1266,13 @@ class BaseConfig(object):
                 self.rootfs_options = vm_drive
                 if not self.fstype in self.vmtypes:
                     self.rootfs_options += ' -no-reboot'
-            self.kernel_cmdline = 'root=%s rw' % (self.get('QB_KERNEL_ROOT'))
+
+            # By default, ' rw' is appended to QB_KERNEL_ROOT unless either ro or rw is explicitly passed.
+            qb_kernel_root = self.get('QB_KERNEL_ROOT')
+            qb_kernel_root_l = qb_kernel_root.split()
+            if not ('ro' in qb_kernel_root_l or 'rw' in qb_kernel_root_l):
+                qb_kernel_root += ' rw'
+            self.kernel_cmdline = 'root=%s' % qb_kernel_root
 
         if self.fstype == 'nfs':
             self.rootfs_options = ''