support board vendor-common makefiles

if a board/$(VENDOR)/common/Makefile exists, build it.

also add the first such case, board/freescale/common/Makefile, to
handle building board-shared EEPROM, PIXIS, and MDS-PIB code, as
dictated by board configuration.

thusly get rid of alternate build dir errors such as:

FATAL: can't create /work/wd/tmp/u-boot-ppc/board/freescale/mpc8360emds/../common/pq-mds-pib.o: No such file or directory

by putting the common/ mkdir command in its proper place (the common
Makefile). Common bits from existing individual board Makefiles have
been removed.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
diff --git a/include/configs/MPC8641HPCN.h b/include/configs/MPC8641HPCN.h
index bf25f7f..7d8a380 100644
--- a/include/configs/MPC8641HPCN.h
+++ b/include/configs/MPC8641HPCN.h
@@ -185,6 +185,7 @@
 #define CFG_OR3_PRELIM		0xfff06ff7	/* 1MB PIXIS area*/
 
 
+#define CONFIG_FSL_PIXIS	1	/* use common PIXIS code */
 #define PIXIS_BASE	0xf8100000      /* PIXIS registers */
 #define PIXIS_ID		0x0	/* Board ID at offset 0 */
 #define PIXIS_VER		0x1	/* Board version at offset 1 */