package/flare-engine: re-enable package on microblaze

With Microblaze Gcc version < 8.x the build hangs due to bug 85180:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85180
To avoid this, the flare-engine package has a
!BR2_TOOLCHAIN_HAS_GCC_BUG_85180 dependency. However, gcc bug 85180 only
triggers when optimization is enabled, so we can work around the issue
by passing -O0, which is what we do in other Buildroot packages to work
around this bug.

So, this commit passes -O0 when BR2_TOOLCHAIN_HAS_GCC_BUG_85180, and
re-enables flare-engine on Microblaze. It is not enough to set
CMAKE_CXX_FLAGS, because flare-engine's CMakeLists.txt sets
CMAKE_CXX_FLAGS_<BUILD_TYPE> (depending on the value of
CMAKE_BUILD_TYPE), and the build-type-specific flags come after the
generic flags, so our -O0 gets overridden again. Therefore, also set
CMAKE_BUILD_TYPE to the dummy value Buildroot.

Fixes:
http://autobuild.buildroot.net/results/706/7065e14917a8bbc0faf21b29183ac55b6c800ee3/



Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
[Arnout: extend explanatory comment and update commit log]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>