erlang: fix comment dependencies

The comment was missing the dependency on BR2_USE_MMU, and was using
'depends on !BR2_TOOLCHAIN_HAS_THREADS && BR2_STATIC_LIBS' while it
should in fact be '!BR2_TOOLCHAIN_HAS_THREADS || BR2_STATIC_LIBS',
since we want show the comment *either* when we don't have threads
*or* when we are building a purely static lib system.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>