Make linking against libgcc configurable

Many (especially ARM) tool chains seem to come with broken or
otherwise unusable (for the purposes of builing U-Boot) run-time
support libraries `libgcc.a'. By using the "USE_PRIVATE_LIBGCC"
setting we allow to use alternative libraries instead.

"USE_PRIVATE_LIBGCC" can either be set as an environment variable in
the shell, or as a command line argument when running "make", i. e.
	$ make USE_PRIVATE_LIBGCC=yes
or
	$ USE_PRIVATE_LIBGCC=yes
	$ export USE_PRIVATE_LIBGCC
	$ make

The value of "USE_PRIVATE_LIBGCC" is the name of the directory which
contains the alternative run-time support library `libgcc.a'. The
special value "yes" selects the directory $(OBJTREE)/lib_$(ARCH) .

Note that not all architectures provide an alternative `libgcc.a' in
their lib_$(ARCH) directories - so far, only ARM does.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Cc: Prafulla Wadaskar <prafulla@marvell.com>
cc: Stefan Roese <sr@denx.de>
diff --git a/board/trab/Makefile b/board/trab/Makefile
index 3a92c0d..2afad88 100644
--- a/board/trab/Makefile
+++ b/board/trab/Makefile
@@ -36,8 +36,6 @@
 
 OBJS_FKT := $(addprefix $(obj),$(COBJS_FKT))
 
-gcclibdir := $(shell dirname `$(CC) -print-libgcc-file-name`)
-
 LOAD_ADDR = 0xc100000
 
 #########################################################################
@@ -52,7 +50,7 @@
 		-L$(obj)../../examples/standalone -lstubs \
 		-L$(obj)../../lib_generic -lgeneric \
 		$(obj)../../lib_arm/div0.o \
-		$(obj)../../lib_arm/_*.o
+		$(PLATFORM_LIBS)
 	$(OBJCOPY) -O srec $(<:.o=) $@
 
 $(obj)trab_fkt.bin:	$(obj)trab_fkt.srec