openssl: disable libressl on uClibc noMMU

uClibc doesn't implement __register_atfork() on noMMU systems, causing
a build failure of libressl. Since openssl is available as an
alternative on such platforms, we just disable libressl on
uClibc/noMMU systems.

Fixes:

/home/thomas/projets/buildroot/output/build/libressl-2.5.5/crypto/.libs/libcrypto.a(arc4random.o): In function `_rs_init.part.1':
arc4random.c:(.text+0x74): undefined reference to `__register_atfork'
collect2: error: ld returned 1 exit status

This build issue is not visible in the autobuilders, because libressl
is only selectable through a choice, which isn't randomized by the
autobuilder logic.

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