runqemu: Add new option to disable vga emulation

When using nographic, explicitly disabling vga is needed some cases
since some qemu firmware (OpenFirmware ) defaults to std vga, and
when vga is enabled then it disables output to serial and redirects
that to vga which is by design, hwoever we expect the console output
to go to serial when using nographic, therefore its important to
disable vga with nographic especially on qemu based ppc platforms

(From OE-Core rev: eabbdb3c4f78ab23348a6b988f59d9d98b5e2c7c)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Cc: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/runqemu

@@ -65,6 +65,7 @@ of the following environment variables (in any order):
   MACHINE - the machine name (optional, autodetected from KERNEL filename if unspecified)
   Simplified QEMU command-line options can be passed with:
     nographic - disable video console
+    novga - Disable VGA emulation completely
     sdl - choose the SDL UI frontend
     gtk - choose the Gtk UI frontend
     gl - enable virgl-based GL acceleration (also needs gtk or sdl options)
@@ -489,6 +490,8 @@ class BaseConfig(object):
             elif arg == 'egl-headless':
                 self.set_dri_path()
                 self.qemu_opt_script += ' -vga virtio -display egl-headless,show-cursor=on'
+            elif arg == 'novga':
+                self.qemu_opt_script += ' -vga none'
             elif arg == 'serial':
                 self.kernel_cmdline_script += ' console=ttyS0'
                 self.serialconsole = True