cobalt/build: pipeline: select IPIPE layer only if present

We may be able to build against a Dovetail-enabled kernel at some
point, so do not force-enable CONFIG_IPIPE, it might not be there.

At this chance, remove obsolete internal switches and conditions. All
I-pipe implementations depend on the GENERIC_CLOCKEVENTS framework,
and support for the legacy I-pipe V1 API is long gone.

No functional change is introduced.
Signed-off-by: Philippe Gerum <rpm@xenomai.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

scripts/Kconfig.frag

 menuconfig XENOMAI
 	depends on X86_TSC || !X86
 	bool "Xenomai/cobalt"
-	select IPIPE
-	select IPIPE_WANT_APIREV_2
+	select IPIPE if HAVE_IPIPE_SUPPORT
+	select IPIPE_WANT_APIREV_2 if IPIPE
 	default y
 	help
 	  Xenomai's Cobalt core is a real-time extension to the Linux
@@ -30,10 +30,6 @@ comment "or CPU idle features is enabled. Any of these options may"
 comment "cause troubles with Xenomai. You should disable them."
 endif
 
-if !GENERIC_CLOCKEVENTS
-comment "NOTE: Xenomai 3.x requires CONFIG_GENERIC_CLOCKEVENTS"
-endif
-
 config XENO_VERSION_MAJOR
        int
        default @VERSION_MAJOR@