buildroot

php: fix selection of interfaces depending on BR2_USE_MMU

Commit c457213fbca5f3493544c8a4335b623620458b0c reworked the selection
of PHP interfaces, moving from a choice..endchoice to a three
independent sub-options.

Thanks to how choices are handled by kconfig, an option suitable for
noMMU platforms was automatically chosen by default. When the
choice...endchoice got removed in
c457213fbca5f3493544c8a4335b623620458b0c, we introduced a "select" for
one of the interfaces, to make sure at least one interface is
selected.

Unfortunately, the selected interface does not build on noMMU
platforms. In order to preserve the previous behavior (CGI interface
is selected by default on MMU capable platforms), this commit makes
the select logic a bit more complicated:

 - CGI is selected on MMU-capable platforms if neither CLI or FPM are
   selected

 - CLI is selected on MMU-less platforms, since this is anyway the
   only available interface.

Fixes:

  http://autobuild.buildroot.net/results/85ea09188f1e996cb14b4b83a17736c096c569a8/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>