package/mono: bump to version 4.8.1.0

This patch bumps mono the version 4.8.1.0. Simultaneously it removes an
upstreamed patch and updates another one to the latest source code.

It also disables the compilation of 'aot' cause the build system was
fixed to support this option again.

It also disables the compilation of the optional BoringTLS stack: this
stack is distributed as an external component inside the mono source
tree and it carries it's own build system (cmake). To be compiled inside
buildroot it requires hacking the mono build system to pass the correct
compiling options to cmake. This will be done in a future patch set.

Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>