* gnu/home/services.scm (shell-variable-definitions): New procedure.
* gnu/home/services/shells.scm (serialize-shell-variables): New procedure.
(home-bash-configuration)[variables]: New field.
(add-bash-configuration): Honor it.
(home-bash-extension)[variables]: New field.
(home-bash-extensions): Handle ‘variables’.
* doc/guix.texi (Shells Home Services): Update. Change ‘PS1’ example to use
‘variables’, not ‘environment-variables’.
* doc/he-config-bare-bones.scm: Use ‘variables’ instead of ‘bash-profile’.
Change-Id: I29083d6313d10b1eb9d91eccacfb33efefe60d58