base-files: prevent issues w/ overlay on powerloss after sysupgrade

base-files: prevent issues w/ overlay on powerloss after sysupgrade

Due to filesystem write caching the old configuration data could stay
out of flash for a long time during a first boot after the sysupgrade.
Power loss during this period could damage the overlay data and even
make device inaccessable via the network.

Fix this by syncing data to a flash as soon as the previous
configuration will be unpacked after the sysupgrade. Also sync the FS
state after the sysupgrade.tgz archive removing to prevent duplicative
extraction of a previous configuration.

Tested with AMD Geode based board.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>