axs101: work-around failing mount message

Afetr switching to SysV init (that by default has 2 consoles - on serial port
and graphic screen) unexpected message started to appear:
=============
mount: mounting / on  failed: No such file or directory
=============

For some reason remounting of / with option "rw" produced mentioned message.
To suppress it we disable remounting of root partiotion which is not required
BTW in our case when initramfs is mounted as "rw" right away.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>