buildroot

system: systemd only really supports a R/W rootfs

Supporting a R/O rootfs needs a bit more love to be fully operational.
This will come in later patches...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>