cmd: bootvx: Always get VxWorks bootline from env

So far VxWorks bootline can be contructed from various environment
variables, but when these variables do not exist we get these from
corresponding config macros. This is not helpful as it requires
rebuilding U-Boot, and to make sure these config macros take effect
we should not have these environment variables. This is a little
bit complex and confusing.

Now we change the logic to always contruct the bootline from
environments (the only single source), by adding two new variables
"bootdev" and "othbootargs", and adding some comments about network
related settings mentioning they are optional. The doc about the
bootline handling is also updated.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Tested-by: Hannes Schmelzer <oe5hpm@oevsv.at>