runqemu: Allow the user to specity no kernel or rootFS

In some cirsumstances the user doesn't want to supply a kernel, rootFS
or DTB to QEMU. This will occur more now that QEMU supports loading
images using a '-device loader' method.

Allow users to specify 'none' for QB_DEFAULT_FSTYPE or QB_DEFAULT_KERNEL
to avoid supplying these options to QEMU.

(From OE-Core rev: 2cc01c4e46b05b7ffcc8a11e7ebde6c43256c3c3)

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/runqemu

@@ -453,7 +453,7 @@ class BaseConfig(object):
     def check_rootfs(self):
         """Check and set rootfs"""
 
-        if self.fstype == 'nfs':
+        if self.fstype == 'nfs' or self.fstype == "none":
             return
 
         if self.rootfs and not os.path.exists(self.rootfs):
@@ -481,6 +481,10 @@ class BaseConfig(object):
         # QB_DEFAULT_KERNEL is always a full file path
         kernel_name = os.path.basename(self.get('QB_DEFAULT_KERNEL'))
 
+        # The user didn't want a kernel to be loaded
+        if kernel_name == "none":
+            return
+
         deploy_dir_image = self.get('DEPLOY_DIR_IMAGE')
         if not self.kernel:
             kernel_match_name = "%s/%s" % (deploy_dir_image, kernel_name)
@@ -858,6 +862,9 @@ class BaseConfig(object):
             k_root = '/dev/nfs nfsroot=%s:%s,%s' % (self.nfs_server, self.nfs_dir, self.unfs_opts)
             self.kernel_cmdline = 'root=%s rw highres=off' % k_root
 
+        if self.fstype == 'none':
+            self.rootfs_options = ''
+
         self.set('ROOTFS_OPTIONS', self.rootfs_options)
 
     def guess_qb_system(self):