tftpd: install TFTP client as well

This patch installs TFTP client as well and overrides BusyBox
implementations if BusyBox is enabled.

[Thomas: fix typo SYSKLOGD_DEPENDENCIES -> TFTPD_DEPENDENCIES noticed
by Baruch.]

Signed-off-by: Kelvin Cheung <keguang.zhang@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>