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/board/freescale/common/pixis.c b/board/freescale/common/pixis.c
index 99cc2ee..ae4bef1 100644
--- a/board/freescale/common/pixis.c
+++ b/board/freescale/common/pixis.c
@@ -27,6 +27,8 @@
 #include <watchdog.h>
 #include <asm/cache.h>
 
+#ifdef CONFIG_FSL_PIXIS
+
 #include "pixis.h"
 
 
@@ -470,3 +472,4 @@
 	"    pixis_reset altbank cf <SYSCLK freq> <COREPLL ratio> <MPXPLL ratio>\n"
 	"    pixis_reset cf <SYSCLK freq> <COREPLL ratio> <MPXPLL ratio>\n"
 	);
+#endif /* CONFIG_FSL_PIXIS */