gcc 11.x fixes

gcc 11.x fixes

Changes for compiling with gcc 11.x:
- g++ now seems to instantiate duplicate entries for the set member
  function in the contextclass template class in binding-wrap.hpp.
  The use of a closure as a default argument value seems to be the
  culprit, as it seems there are longstanding issues with respect to
  using closures like that and resulting symbol names (i.e. the use
  of a closure isn't necessarily recognized as generating unique
  instantiations).  In theory, C++17 should explicitly allow this
  when the closure has no captures, but bumping up to -std=gnu++17
  did not fix the issue.  To avoid it, replace the closure usage
  with a private static member function.
- In afb-hook.c, tweaked the ignoring of the writev return code to
  make the stricter checking in gcc 11 happy.
- In decode_base64 in wrap-json.c, initialize u16 as gcc now seems
  to miss that it will be initialized on the first loop iteration.

Bug-AGL: SPEC-3819

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I8876db196b46cc35ecd2798e20d5ec3425df865e