or1k: Build Infrastructure

Here we define the minumum linux kernel version at 5.4.0, as that is the
long term support version where 32-bit architectures start to support
64-bit time API's.  The OpenRISC kernel had some bugs up until version 5.8
which caused issues with glibc fork/clone, they have been backported to
5.4 but not previous versions.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>