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/mpc8544ds/Makefile b/board/freescale/mpc8544ds/Makefile
index 308f707..006fdc9 100644
--- a/board/freescale/mpc8544ds/Makefile
+++ b/board/freescale/mpc8544ds/Makefile
@@ -24,14 +24,9 @@
 
 include $(TOPDIR)/config.mk
 
-ifneq ($(OBJTREE),$(SRCTREE))
-$(shell mkdir -p $(obj)../common)
-endif
-
 LIB	= $(obj)lib$(BOARD).a
 
-COBJS	:= $(BOARD).o \
-	   ../common/pixis.o
+COBJS	:= $(BOARD).o
 
 SOBJS	:= init.o