ipmiutil: only make available for x86

IPMI is a Intel/PC spec, so it doesn't make much sense to build on !x86 -
And ipmiutil uses sys/io.h and inb()/outb() which isn't supported on all
architectures, so limit to x86 like we do for acpid.

Fixes:
http://autobuild.buildroot.net/results/b2b/b2b50be359431b828e60c2ffdcd0fa1ca1cd5605/
http://autobuild.buildroot.net/results/f63/f63a25d6faca407da7332b2806300baadbe33326/
http://autobuild.buildroot.net/results/92f/92f54495f7f2a4b70aff18c094baa71adb0f5985/
http://autobuild.buildroot.net/results/f21/f21958c19f145baf83ebbb02b8526f1a58ea25ec/



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>