recipetool: create: avoid extra blank lines in output recipe

If we output extra blank lines (because of some automated editing) then
it makes the output recipe look a bit untidy. You could argue that we
should simply have the editing code not do that, but sometimes we don't
have enough context there for that to be practical. It's simple enough
to just filter out the extra blank lines when writing the file, so just
do it that way.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>