[build] Remove previous image before mke2fs

Prevents a warning.

Change-Id: I8f0e422ea65387479c9b730df55a6b6db41a9b94

build.sh

diff --git a/build.sh b/build.sh
index d066ad1..4f25ec1 100755
--- a/build.sh
+++ b/build.sh
@@ -34,4 +34,5 @@
 declare -r ADDITIONAL_BLOCKS=1024
 declare -r SIZE=$(du -sb ${OUT_DIR} | grep -o '^[0-9]*')
 declare -r BLOCKS=$(($((${SIZE}/${BLOCK_SIZE}))+${ADDITIONAL_BLOCKS}))
+rm -f "${IMAGE_FILE}"
 mke2fs -q -d "${OUT_DIR}" -t ext2 -b ${BLOCK_SIZE} "${IMAGE_FILE}" ${BLOCKS}