configs: Drop '$(ARCH)' usage in CONFIG_SPL_LDSCRIPT

In a few boards we had overridden, intentionally, the value used for
CONFIG_SPL_LDSCRIPT.  However, rather than using the ARCH value (arm)
they used the $(ARCH) variable in make.  This doesn't help really, so
switch to a hard-coded value.

Signed-off-by: Tom Rini <trini@konsulko.com>