BaseTools: Enable the flag to treat dynamic pcd as dynamicEx

BaseTools: Enable the flag to treat dynamic pcd as dynamicEx

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1688

In order to support binary build, build tool add a
flag to convert type of Dynamic Pcd to DynamicEx Pcd

User can append -D PCD_DYNAMIC_AS_DYNAMICEX to build command
to enable this function.
Also, user can add "PCD_DYNAMIC_AS_DYNAMICEX = TRUE/FALSE" to the
defines section of Dsc file to enable this function.
PCD_DYNAMIC_AS_DYNAMICEX is a new reserved key word for this function.

Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Yuwei Chen <yuwei.chen@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>