modules: added 'finish' module.

This module does those little things that are required to ensure that the
target system works correctly, but that don't warrant an entire module for
themselves.

Currently the only job that this module does is to update the initramfs
image.
This fixes the boot-up issues found on Semplice 7 on freshly installed systems.
modules/fstab: raise the ZRAM_LIMIT barrier to 4096 megabytes.

zRAM is useful on systems that have 2 to 4 GB of physical memory installed, so
it's wise to configure it for those machines too.
modules/fstab: do not refresh the initramfs anymore.

The new 'finish' module introduced in ca8a401 handles that job now,
so avoid doubling the work to do.