Log Message:
fix build of u-boot using pylibfdt on macOS.

u-boot.mk:
  pass PYTHON2, PYTHONCONFIG and PYTHONLIBPATH in ALL_ENV
patch-Makefile:
  make PYTHON2 overridable.  Add PYTHONCONFIG and BYTHONLIBPATH.
patch-lib_bch.c:
  #include <libkern/OSByteOrder.h> and use OSSwapHostToBigInt32
  as in u-boot master ab8fc41302 "fix compile error on macOS Mojave".
patch-scripts_dtc_pylibfdt_Makefile:
  determine PYTHONCLFAGS and PYTHONLIBS by asking PYTHONCONFIG.
  build PYTHONLDFLAGS from PYTHONLIBPATH and PYTHONLIBS
  add PYTHON{LD.C}FLAGS to LDFLAGS and CPPFLAGS in cmd_pymod.

bump PKGREVISION

tested with u-boot-nanopi-mi.