erlang: add option to enable SMP support

Erlang builds two emulators depending on whether SMP is supported or
not. If it is known that the target does not support SMP, turning off
the SMP emulator saves compile time and reduces the size of the
installation.

Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>