Get rid of drivers class

The use of ramstage.a required the build system to handle some
object files in a special way, which were put in the drivers
class.

These object files didn't provide any symbols that were used
directly (but only via linker magic), and so the linker never
considered them for inclusion.

With ramstage.a gone, we can drop this special class, too.

Change-Id: I6f1369e08d7d12266b506a5597c3a139c5c41a55
Signed-off-by: Patrick Georgi <patrick.georgi@secunet.com>
Reviewed-on: http://review.coreboot.org/1872
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/cpu/intel/model_f4x/Makefile.inc

@@ -1 +1 @@
-driver-y += model_f4x_init.c
+ramstage-y += model_f4x_init.c