fix(sptool): operators "is/is not" in sp_mk_gen.py

Replace the "is/is not" operator by "==/!=" for literals, to fix the
syntax warnings below:

tools/sptool/sp_mk_generator.py:93: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  return len(sppkg_rule) is not 0

tools/sptool/sp_mk_generator.py:203: SyntaxWarning: "is" with a literal. Did you mean "=="?
  assert(len(uuid_lines) is 1)

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: I10800f6b607942542aa2cbaaecac86b854f6b56a