system: expose if system was booted from initramfs

system: expose if system was booted from initramfs

It can be good for UI to show to the user that the system was booted
from initramfs ie. no writable permanent storage is available.
I imagine LuCI only serving applications which are explicitely marked
as being shown even in initramfs mode, ie. nothing but status,
network->interfaces, network->wireless, system->upgrade,
system->backup, system->backuprestore tabs.
Also sysupgrade could take into account we are running on initramfs
and perform offline backup/restore of whatever is in the flash.
In that way OpenWrt-generated initramfs-images can serve as recovery
OS on devices with dual-boot in a meaningful way.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>