* gnu/build/activation.scm (activate-firmware): New procedure.
* gnu/system.scm (<operating-system>)[firmware]: New field.
(directory-union): New procedure.
(%base-firmware): New variable.
(operating-system-activation-script): Use 'directory-union', and call
'activate-firmware'.
* doc/guix.texi (operating-system Reference): Document 'firmware'.