base-files: clean up nand tar sysupgrade code

base-files: clean up nand tar sysupgrade code

Prepares code for ubirename-based safe sysupgrade implementation.

Fixes several issues:
- the special CI_KERNPART value "none" is ignored if an MTD partition
  named "none" exists
- misleading variable names (such as has_kernel to mean "tar has kernel
  and it should not be written to an MTD partition but a UBI volume")
- inconsistent treatment of zero-length tar member files
- inconsistent meaning of "0" and "" variable values
- redundant operations (unneeded untaring, repeated untaring, unneeded
  partition lookups)
- inconsistent variable quoting

Signed-off-by: Rodrigo Balerdi <lanchon@gmail.com>