Updated picoTCP support, based on newer HTTPS demo

Hello,

we updated the support for PICOTCP and PICOTCP_DEMO, in order to be able to compile our HTTPS public demo, using CyaSSL + picoTCP. 

See https://github.com/tass-belgium/cyassl-picotcp  which is currently broken, until this pull request gets accepted.

Changes:
- FreeRTOS no longer needed
- picotcp-bsd library no longer needed
- some flags added for demo compilation under CYASSL_PICOTCP_DEMO

TIA, 

/picoTCP team