package/openrc: remove keymaps units if kbd package is not selected

keymaps and save-keymaps require kbd_mode and dumpkeys, respectively, so
remove them if the kbd package is not selected (e.g. devices with serial
console, only).

Signed-off-by: Carlos Santos <unixmania@gmail.com>
Tested-by: Adam Duskett <aduskett@gmail.com>
[yann.morin.1998@free.fr:
  - expand to three commands to match the existing hook
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>