net: Remove env_get_ip helper() function

net: Remove env_get_ip helper() function

Currently, we have the function env_get_ip which takes an IP address
in string form and returns a struct in_addr representation of that
address. It is however little used and means that a number of places
indirectly (and unclearly) get <env.h> via <net.h>. To clean this up
start by replacing env_get_ip() calls with string_to_ip() calls. This is
generally a no-op as env_get_ip(str) is an inline of
string_to_ip(env_get(str)) but in a few cases we can or already have
stored the result of env_get(str) and can save the additional call.

Signed-off-by: Tom Rini <trini@konsulko.com>