cfgfile: support global properties section

cfgfile: support global properties section

The current implementation of the cfgfile library requires that all
key=value pairs be within [SECTION] definitions.  The ini file standard
allows for key=value pairs in an unnamed section.

   https://en.wikipedia.org/wiki/INI_file#Global_properties

This commit adds the capability of parsing key=value pairs from such an
unnamed section. The CFG_FLAG_GLOBAL_SECTION flag must be passed to the
rte_cfgfile_load() API to enable this functionality.  Any key=value pairs
found before the first section can be accessed in the section named
"GLOBAL".

Signed-off-by: Allain Legacy <allain.legacy@windriver.com>
Acked-by: Cristian Dumitrescu <cristian.dumitrescu@intel.com>