BaseTools/build: Set MakefileName

BaseTools/build: Set MakefileName

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3653

Running the following command:
  python3 build/build.py -a AARCH64 -t GCC5
  -p ArmPlatformPkg/ArmPlatformPkg.dsc -b DEBUG libraries
triggers the following error:
  make: *** Build/ArmPlatform/DEBUG_GCC5/AARCH64/MdePkg/Library/
  BasePcdLibNull/BasePcdLibNull: Is a directory.  Stop.

Indeed, MakefileName is set to en empty string. Setting MakefileName
resolves the error.

Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Reviewed-by: Chris Jones <christopher.jones@arm.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>