[script] Allow initial whitespace on lines containing labels

Initial whitespace is already accepted on lines containing commands,
since it gets ignored by the system() call.  Minimise surprise and
allow for neater indentation of scripts by also allowing whitespace on
lines containing labels.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[script] Allow commands following a script label

Allow commands to be placed on the same line as a label.  This allows
for improved legibility of loop constructions by incorporating the
loop check condition into the same line as the loop label.  For
example, to iterate over network devices using the forthcoming "inc"
command:

  set idx:int16 0
  :loop isset ${net${idx}/mac} || goto loop_done
    echo net${idx} is a ${net${idx}/chip} with MAC ${net${idx}/mac}
    inc idx && goto loop
  :loop_done

Signed-off-by: Michael Brown <mcb30@ipxe.org>