Restore compatiblity with Bash 4
The title says it all, this MR aims at restoring the compatibility of the Makefile with Bash 4. This is somewhat needed to run on gem904atca1
(CentOS 7).
This is achieved by simplifying the register generation step in Makefile. Easier-to-follow conventions have also been used (flavor, station,...)
Fixes #53 (closed)