It would seem that either we need to require the periph_pm module in shell
or make it optional... since we have many other optional modules here
and we still may want the RIOT_VERSION command, lets make it optional for now.
This allows to define an extra string that will be output as part
of the version command.
e.g. an application may define
RIOT_VERSION_EXTRA += v1.33.7
RIOT_VERSION_EXTRA += flashed by $(shell whoami
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.
