oeqa.buildperf: functionality to drop kernel caches

Add a new utility class for dropping Linux kernel caches.  It uses sudo
and tee to write to the drop_caches file. Checking if the user has the
permissions to drop caches (without a password) is done by trying to
writing an invalid value to the drop_caches file. This way, we will find
if writing (with tee) is possible but not really dropping caches, yet.

User can avoid giving the password by adding something like:
<user> ALL = NOPASSWD: /usr/bin/tee /proc/sys/vm/drop_caches
to the system sudoers file.

(From OE-Core rev: c9cb248429ced50c96d11ba5361c272d4c9b9323)

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oeqa/buildperf/__init__.py

@@ -10,3 +10,4 @@
 # more details.
 #
 """Build performance tests"""
+from .base import KernelDropCaches