kernel: enable open by fhandle syscalls

This is needed by many services to function properly and as
all modern distributions got it enabled, it starts to be a
de-facto standard, i.e. user-space starts to silently depend
on it.

This also pulls in EXPORTFS, however, the kernel binary size
increases only a little.
On ARM systems comes down to 800 bytes uncompressed and about
200 bytes compressed size.
On MIPS systems it's about 1.2 kB size increase of the LZMA
compressed kernel.

v2: use menuconfig option instead of just enabling the option

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44765 3c298f89-4303-0410-b956-a3cf2f4a3e73