BaseTools/PlatformAutoGen: MAKE_FLAGS and MAKE_PATH fixes

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3312



Update parsing of MAKE_FLAGS in DSC [BuildOptions] sections
to split the flags into a list to be compatible with
running the make command using Popen().  Parsing MAKE_FLAGS
from tools_def.txt already uses _SplitOption().  This change
uses the same _SplitOption() method for MAKE_FLAGS from a
DSC [BuildOptions] section.

Also update the parsing of MAKE_PATH to support MAKE_PATH
from tools_def.txt or the DSC [BuildOptions] section.  MAKE_PATH
in DSC [BuildOptions] section is higher priority than MAKE_PATH
in tools_def.txt.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Yuwei Chen <yuwei.chen@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Yuwei Chen <yuwei.chen@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>