process consecutive commands

Process consecutive commands separated by null-termations. Since
it is a FIFO, in theory, two commands can be queued from two
independent calls to psplash-write. This also makes the command
parser more robust. With this code, sequences like this get
parsed just fine:
  echo -e "\nPROGRESS 10\n\0\nQUIT" > /run/psplash_fifo

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>