build_boot.sh: remove check for obsolete target header

The C sources themselves now complain when the build configuration is
not complete.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>
diff --git a/build_boot.sh b/build_boot.sh
index 72ee96f..d906016 100755
--- a/build_boot.sh
+++ b/build_boot.sh
@@ -8,11 +8,6 @@
 	exit 1;
 fi
 
-if [ ! -f "$(dirname $0)/boot/zephyr/targets/${BOARD}.h" ]; then
-	echo "Board $BOARD not yet supported, please use a supported target."
-	exit 1;
-fi
-
 # Check if there is a valid Zephyr environment available
 if [ -z "$ZEPHYR_BASE" ]; then
 	echo "ZEPHYR_BASE not provided by the environment."