bootstrap: fix shellcheck warnings

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ic553996a3bedf6c45703bc2dec703b3532f63762
diff --git a/bootstrap.sh b/bootstrap.sh
index 50b75b7..95572de 100755
--- a/bootstrap.sh
+++ b/bootstrap.sh
@@ -8,11 +8,12 @@
     clean)
         test -f Makefile && make maintainer-clean
         for file in ${AUTOCONF_FILES}; do
-            find -name "$file" | xargs -r rm -rf
+            find . -name "$file" -prune -exec rm -rf {} \;
         done
         exit 0
         ;;
 esac
 
 autoreconf -i
+# shellcheck disable=SC2016
 echo 'Run "./configure ${CONFIGURE_FLAGS} && make"'