Clean up Makefile/Kconfig dependency checks

Some of the application Makefiles have configuration checks that overlap dependencies given in Kconfig and Make.defs.
Ideally each check should be done at only one place, and dependencies should be handled by Kconfig.

Note that enabling *partial* functionality based on config checks in Makefile is fine, but if the whole application depends on some configuration item, that should be a dependency in Kconfig.

Makefiles that have a duplicate check for `CONFIG_appname` in both Make.defs and Makefile:

* fsutils/ipcfg
* fsutils/mkfatfs
* fsutils/mksmartfs
* fsutils/passwd
* netutils/dhcp6c
* netutils/ping

Makefiles that check for some other config item such as `CONFIG_NET_UDP`. In most cases the dependency is already in Kconfig and the Makefile check can be removed, in other cases dependency should be added to Kconfig:

* netutils/dhcpc
* netutils/dhcpd
* netutils/discover
* netutils/ftpc
* netutils/ftpd
* netutils/ntpclient
* netutils/smtp
* netutils/telnetc
* netutils/tftpc
* netutils/thttpd
* netutils/webclient
* netutils/webserver
* netutils/xmlrpc