Currently it is not possible to change symbolic links in the default
environment. For example if the default environment contains the
link "current_boot -> mmc" then the following does not work:

$ rm current_boot
$ ln net current_boot
$ saveenv

After reset the link is still pointing to "mmc" and this error appears:

"symlink: /env/boot/current_boot -> net : error -17"

The patch fixes the above problem and moves the "ENV_FLAG_NO_OVERWRITE"
to enable handling of symbolic links in this case too.

Signed-off-by: Zahari Doychev <zahari.doychev@linux.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>