Makefile: add .DEFAULT target to further guide new users

[mguetschow]

Contribution description

With #20187, running make or make all in the RIOT base directory prints the welcome message including a list of available targets to be called from there. However, when the user enters an unknown target such as make unknown, Make only outputs a generic error.

This PR fixes this and shows the welcome message instead.

Testing procedure

on master:

$ make unknown
make: *** No rule to make target 'unknown'.  Stop.

with this PR:

$ make unknown         
*** ERROR: unrecognized target "unknown"

Welcome to RIOT - The friendly OS for IoT!

You executed 'make' from the base directory.
Usually, you should run 'make' in your application's directory instead.

Please see our Quick Start Guide at:
    https://doc.riot-os.org/getting-started.html
You can ask questions or discuss with other users on our forum:
    https://forum.riot-os.org

Available targets for the RIOT base directory include:
 generate-{board,driver,example,module,pkg,test,features}
 info-{applications,boards,emulated-boards} info-applications-supported-boards
 print-versions
 clean distclean pkg-clean
 doc doc-{man,latex}

==> tl;dr Try running:
    cd examples/default
    make BOARD=<INSERT_BOARD_NAME>
make: *** [Makefile:83: unknown] Error 1

Issues/PRs references

#20187

[riot-ci]

Murdock results

✔️ PASSED

ec9cd25 Makefile: add .DEFAULT target to further guide new users

Success	Failures	Total	Runtime
10196	0	10197	17m:40s

Artifacts

• Documentation preview