libpayload: Fix ARM workaround code

_LDFLAGS+="foo" did not work in my shell (bash on Ubuntu 15.10),
so change it to _LDFLAGS="$_LDFLAGS foo". I'm mildly surprised
that this ever worked.

Change-Id: I59c10f34992240c6df2ec7f24aebc6daafb76493
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: https://review.coreboot.org/14076
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

payloads/libpayload/bin/lpgcc

@@ -184,7 +184,7 @@ else
 
     if grep -q ARM64_A53_ERRATUM_843419=y $BASE/../libpayload.config &&
        grep -q fix-cortex-a53-843419 $BASE/../libpayload.xcompile; then
-            _LDFLAGS+=" -Wl,--fix-cortex-a53-843419"
+            _LDFLAGS="$_LDFLAGS -Wl,--fix-cortex-a53-843419"
     fi
 
     if [ $DEBUGME -eq 1 ]; then