Make gcc-built lwIP default one

[igrr]

In SDK 1.5.3 lwIP source code in included with the SDK release, which is a good indication that future SDK versions will include that as well. This issue tracks switching from Espressif-provided xcc-built lwIP to gcc-built one.

• Remove workarounds made in ClientContext to work around return value passing bug
• Update lwIP source code from the latest SDK and build new binary
• Update boards.txt to use gcc built lwip by default

This will enable us to fix #1924, #1826, #1505, #1050, #1262.

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[andig]

@igrr fyi- I remember you metioning that gcc code size is bugger, however according to my tests it became smaller when switching lwip.