* update to lwIP-2.1.0rc1: partial SACK support
fix#4176
* hash fix
* get some flash back due to mistake in conf (fragmentation & reassembly was incorrectly enabled)
(ahah I scared you)
* add missing include files
* update to lwip-2.1.0(release) + remove unused lwIP's include files
* lwIP release 2.1.0, SACK is now default, bigger, no-SACK is selectable
* fix ldscript
* pio
* rename 'sack' option to 'feat'ure option, + IP fragmentation/reassembly
* merge, fix pio
* change internal/hidden string
* pio: more lwip2 configuration: + without sack for no change in flash footprint
