BaseTools/GenFds: Support FDF sections in any order

BaseTools/GenFds: Support FDF sections in any order

https://bugzilla.tianocore.org/show_bug.cgi?id=141

This patch updates EDK II FDF parser in GenFds to allow sections
to be placed in any order in the FDF file.

Cc: Kelly Steele <kelly.steele@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>