EmulatorPkg: Use UEFI Shell by default

Now we will build the UEFI shell from source and include it in the
EmulatorPkg build.

USE_OLD_SHELL can be defined on the command line to force using the
old shell.

Fixes: https://github.com/tianocore/edk2/issues/60
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Cc: Andrew Fish <afish@apple.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>