erlang: Remove BR2_TOOLCHAIN_HAS_THREADS dependency

Erlang can be built without threads support, by passing an
appropriate config option.

This commit drops the config dependency on BR2_TOOLCHAIN_HAS_THREADS
and instead passes --disable-threads if the toolchain doesn't support
threads.

While here, change the wording of the config comment to "dynamic
library" to be consistent with similar comments elsewhere.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>