plat_property_contexts: use fc_sort vs sort

sort respects locale settings, so the value of LC_ALL can affect
how sort orders things. Issues have surfaced when CTS build
servers locale differs from image build server locale. And thus
the prologue of property_contexts differs with what CTS was
expecting.

More information on locale and sort can be found via:
  * locale(1) - man 1 locale
  * sort(1) - man 1 sort
  * https://debbugs.gnu.org/cgi/bugreport.cgi?bug=28539



Rather than deal with this locale setting use fc_sort on
property contexts. This also has the side-effect of
stripping comments, and thus sed can be dropped.

Test: This was tested by:
  * comparing outputs to previous runs
  * booting the x86-64 emulator

Change-Id: I144ef549cc11d9c61849ffc0e1b1b000f1b8d1a8
Signed-off-by: William Roberts <william.c.roberts@intel.com>