[SIMB-1898] simetbox-openwrt-base: more low-memory paranoia

[SIMB-1898] simetbox-openwrt-base: more low-memory paranoia

Try even harder to free memory so that the configuration save process
has a better chance of working.  Also, attempt to detect issues with the
configuration tarball before proceeding.

Much of this should go upstream (modified): sysupgrade really needs to
be a lot more paranoid instead of just "hoping" things will go well.

We stop (and not nicely) any ongoing SIMET measurements early,
otherwise it is quite possible some 32MiB boxes would never manage to
update.