Auto remove any leftover build/ directory

Signed-off-by: David Goulet <dgoulet@torproject.org>

build.sh

@@ -76,6 +76,10 @@ done
 BUILD="$(pwd)/build"
 SIGS_DIRPATH="$(pwd)/sigs"
 
+# Cleanup previous build directory since we need to start from scratch. Prompt
+# before removal just in case since rm -rf are scary.
+runcmd rm -rf -I "$BUILD"
+
 # Create the directory hierarchy we need.
 runcmd mkdir -p "$BUILD"
 runcmd mkdir -p "$SIGS_DIRPATH"