boards: x86: minnowboard: Unset CONFIG_X86_NO_SPECTRE_V4 by default

According to INTEL-SA-00115[1], Intel(r) Atom(tm) Processor E Series
are affected.  All Minnowboards sports one of these processors.

[1]: https://goo.gl/CKyLQy



Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>