Make getenv_IPaddr() global

There are boards out there that do not have network support in
U-Boot (CONFIG_CMD_NET not set), but they do so in Linux. This
makes it desirable to be able to port network configuration (like
the IP address) to the Linux kernel.

We should not make the passing of the IP configuration to Linux
dependent on U-Boot features / settings.

For this, make getenv_IPaddr() global. This fixes build error

u-boot/lib_xxx/board.c:360: undefined reference to `getenv_IPaddr'

on various architectures.

Signed-off-by: Dirk Behme <dirk.behme@googlemail.com>
Acked-by: Ben Warren <biggerbadderben@gmail.com>