ppc4xx: Add custom linker script to board/*/config.mk

These boards have special linker scripts right now. We can't use the
common 4xx linker script here. So overrride the linker script (LDSCRIPT)
in board/*/config.mk and choose the board specific version.

Signed-off-by: Stefan Roese <sr@denx.de>
diff --git a/board/amirix/ap1000/config.mk b/board/amirix/ap1000/config.mk
index c09783a..09c6efa 100644
--- a/board/amirix/ap1000/config.mk
+++ b/board/amirix/ap1000/config.mk
@@ -25,3 +25,6 @@
 # like it's not in RAM.  This is a bit of voodoo to allow it to be
 # run from RAM instead of Flash.
 TEXT_BASE = 0x08000000
+
+# Use board specific linker script
+LDSCRIPT := $(SRCTREE)/board/amirix/ap1000/u-boot.lds