skin_engine: rework the parser to be closer to the langauge grammar.
The parser was unconditionally scanning things which it thought
were conditional/enum lists (or tag arg lists) when they couldn't
possibly be (i.e < inside a param which should be valid).
This change fixes it (i.e %?and(%if(%pv, <, -50), %if(%mp, > 1))
is perfectly valid now.
This *may* break your exsiting skins if you were using %if with < or >
Change-Id: Ibcb42bc6bb78908f79de024b61276b91b1ce02a0
Reviewed-on: http://gerrit.rockbox.org/214
Reviewed-by: Thomas Martitz <kugel@rockbox.org>