llext-edk: export board information to EDK files

Export the currently used board name and (when applicable) qualifiers
and revisions to the EDK files.  This information can be used by EDK
users who deal with multiple targets to differentiate between them.

Uses the existing 'zephyr_string' function to sanitize the strings
instead of a custom regex replace.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>