utils/mtd-utils: flash_eraseall must depend on flash_erase

Since a8801d8665bcc94c63a798e291a03c88f2af0215 upstream commit
flash_eraseall is a shell wrapper around flash_erase.

Signed-off-by: Paul Fertser <fercerpav@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/packages@35004 3c298f89-4303-0410-b956-a3cf2f4a3e73