Temporarily disable Boost.Context library build as it breaks MinGW builds by bringing in a dependency on MASM (The Microsoft Assembler)
Eeschema block end handler code improvements.

* Merge HandleBlockEnd() and HandleBlockEndByPopUp() into as single block
  end function HandleBlockEnd().
* Replace all instances of HandleBlockEndByPopUp() with HandleBlockEnd().
* Change block hot keys to send command events instead of directly calling
  end block handler.
Add PDF output to Pcbnew plot dialog. (fixes 1p:1188972)