kbuild: remove "*_felconfig" target

This target was added by commit cbdd9a97 (sunxi: kconfig: Add
%_felconfig rule to enable FEL build of sunxi platforms.).

At that time, U-Boot used separate .config files for U-Boot proper
and SPL.  I understood the pain to modify both .config and
spl/.config.

Now, we have switched to single .config configuration.
It seems acceptable to run "make menuconfig" or friends to enable
CONFIG_SPL_FEL, as we do for other CONFIGs.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Ian Campbell <ijc@hellion.org.uk>
Cc: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Simon Glass <sjg@chromium.org>