pxe: allow compilation when !defined(CONFIG_CMD_NET)

pxe.c provides both the "pxe" command which relies on a network, and the
"sysboot" command which doesn't. Fix the file to compile when network
support isn't enabled. This is useful e.g. on the Raspberry Pi which has
no network support yet, but will soon support the sysboot command.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>