perf: rename perf_3.4 to perf

perf: rename perf_3.4 to perf

There's nothing kernel-version-specific about the perf_3.4 recipe, so
it's actually misnamed and misleading now that it also gets used with
the 3.8 kernel.

Since the recipe isn't tied to a specific PV, and simply uses
whatever's in STAGING_KERNEL_DIR, there's no reason to add anything
else either to the bare PN, so just use that as the recipe name.

(From OE-Core rev: 9a249e074f0329ad54848b84536e5b7cb117ee2c)

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>