etc: add config files for libubootenv

This allows one to use libubootenv and hence fw_printenv and fw_setenv to modify the U-Boot environment from inside a running linux. For example, this eliminates the need to use a serial console to setup reform-init.

fw_printenv and fw_setenv are contained in the Debian package libubootenv-tool.

/etc/u-boot-inital-env contains variables the environment will be populated with when no environment exists in the eMMC yet. This means this file must be somewhat in sync with the environment compiled into U-Boot. Otherwise, the first use of libubootenv might make your system unbootable or have other strange side effects.