runqemu: Allow the user to override the device tree option

Update the runqemu script to allow the user to specify a device tree
to boot when calling runqemu.

This involves creating a seperate check_dtb() function incase the user
has specified 'none' for the kernel but still wants a device tree.

(From OE-Core rev: 867ac1370b294bfd1ee31f94abb63688f77081a1)

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

scripts/runqemu

@@ -69,6 +69,7 @@ Usage: you can run this script with any valid combination
 of the following environment variables (in any order):
   KERNEL - the kernel image file to use
   ROOTFS - the rootfs image file or nfsroot directory to use
+  DEVICE_TREE - the device tree blob to use
   MACHINE - the machine name (optional, autodetected from KERNEL filename if unspecified)
   Simplified QEMU command-line options can be passed with:
     nographic - disable video console
@@ -178,6 +179,7 @@ class BaseConfig(object):
         self.env_vars = ('MACHINE',
                         'ROOTFS',
                         'KERNEL',
+                        'DEVICE_TREE',
                         'DEPLOY_DIR_IMAGE',
                         'OE_TMPDIR',
                         'OECORE_NATIVE_SYSROOT',
@@ -579,7 +581,7 @@ class BaseConfig(object):
                 raise RunQemuError("Can't find OVMF firmware: %s" % ovmf)
 
     def check_kernel(self):
-        """Check and set kernel, dtb"""
+        """Check and set kernel"""
         # The vm image doesn't need a kernel
         if self.fstype in self.vmtypes:
             return
@@ -608,8 +610,18 @@ class BaseConfig(object):
         if not os.path.exists(self.kernel):
             raise RunQemuError("KERNEL %s not found" % self.kernel)
 
+    def check_dtb(self):
+        """Check and set dtb"""
+        # Did the user specify a device tree?
+        if self.get('DEVICE_TREE'):
+            self.dtb = self.get('DEVICE_TREE')
+            if not os.path.exists(self.dtb):
+                raise RunQemuError('Specified DTB not found: %s' % self.dtb)
+            return
+
         dtb = self.get('QB_DTB')
         if dtb:
+            deploy_dir_image = self.get('DEPLOY_DIR_IMAGE')
             cmd_match = "%s/%s" % (deploy_dir_image, dtb)
             cmd_startswith = "%s/%s*" % (deploy_dir_image, dtb)
             cmd_wild = "%s/*.dtb" % deploy_dir_image
@@ -678,6 +690,7 @@ class BaseConfig(object):
         self.check_rootfs()
         self.check_ovmf()
         self.check_kernel()
+        self.check_dtb()
         self.check_biosdir()
         self.check_mem()
         self.check_tcpserial()