Previously `shell_commands` was a "catch-all" module that included
shell commands for each and every used module that has a shell
companion. Instead, the new `shell_cmds` module is now used to provide
shell commands as individually selectable submodules, e.g.
`cmd_gnrc_icmpv6_echo` now provides the ICMPv6 echo command (a.k.a.
ping).
To still have a "catch all" module to pull in shell commands of modules
already used, `shell_cmds_default` was introduced. `shell_commands`
depends now on `shell_cmds_default` for backward compatibility, but
has been deprecated. New apps should use `shell_cmds_default`
instead.
For a handful of shell commands individual selection was already
possible. Those modules now depend on the corresponding `cmd_%` module
and they have been deprecated.
