openssl: support building the binary without MMU

The commit 720893b6 "openssl: disable
apps for NOMMU" prevented the openssl binary from being built without
MMU in order to fix a build failure without fork(). However, openssl is
designed to support the lack of fork() with -DHAVE_FORK=0, so allow the
openssl binary to be enabled without MMU thanks to this option.

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