uboot-envtools: add support for multiple config partitions

uboot-envtools: add support for multiple config partitions

Most (all?) of the realtek devices have two u-boot config partitions
with a different set of variables in each. The U-Boot shell provides
two sets of apps to manipulate these:

 printenv- print environment variables
 printsys- printsys - print system information variables
 saveenv - save environment variables to persistent storage
 savesys - savesys - save system information variables to persistent storage
 setenv  - set environment variables
 setsys  - setsys  - set system information variables

Add support for multiple ubootenv configuration types, allowing
more than one configuration file.

Section names are not suitable for naming the different
configurations since each file can be the result of multiple sections
in case of backup partitions.

Signed-off-by: Bjørn Mork <bjorn@mork.no>