Since this part is inside "define" statement, double $ should be
used to refer to internal functions, to call the patsubst when
a define is called (rather than defined).

This should fix dependency generation for images/
Reviewed-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>