drm-lease: Disable weston TTY switch

drm-lease: Disable weston TTY switch

When running multiple instances of weston using the drm-backend,
TTY switching should be disabled, so that both instances can
be displayed at the same time.

Backport patches from weston upstream that will disable TTY
switching when either:
  * Any seat other than seat0 is used (multi-seat)
  * There are no real TTY devices available (often true when
    running in a container)

Also, modify the layer priority and SRC_URI append style to make
these patches apply after the AGL appfw layer to avoid patch
fuzz warnings during build.

Bug-AGL: SPEC-3730

Change-Id: Ie9bed50b1c3f60129b1efae95aa77bfcea45f568
Signed-off-by: Damian Hobson-Garcia <dhobsong@igel.co.jp>