BUILD: makefile: rename "linux2628" to "linux-glibc" and remove older targets

BUILD: makefile: rename "linux2628" to "linux-glibc" and remove older targets

The linux targets have become more than confusing over time. We used to
have "linux2628" to match the features available in kernels 2.6.28 and
above, without consideration for the libc, and due to many new features
appearing later in kernels, some other options were added that are not
enabled by default in linux2628, so this target doesn't make any sense
anymore. The older ones (linux 2.2, linux 2.4, ...) do not make sense
either since these versions are not supported anymore. Let's clean things
up by creating a new "linux-glibc" target that matches what is available
by default on Linux kernels and glibc present on supported distros at the
time of release. Other libc implementation may use a custom or generic
target or be added later if needed.

All the older linux targets were removed.