boot: Update tests around network symbols in BOOT_DEFAULTS_CMDS

boot: Update tests around network symbols in BOOT_DEFAULTS_CMDS

As exposed by "make randconfig", we have an issues around a number of
symbols in BOOT_DEFAULTS_CMDS. Due to the nature of how we currently
handle other networking related command options, we need to be testing
for "!NO_NET" (which is the symbol for no networking stack) or "NET ||
NET_LWIP" rather than CMD_NET alone. For consistency and clarity here
use "CMD_NET && !NO_NET" here.

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