devtools: add checks for ABI symbol addition

devtools: add checks for ABI symbol addition

Recently, some additional patches were added to allow for programmatic
marking of C symbols as experimental.  The addition of these markers is
dependent on the manual addition of exported symbols to the EXPERIMENTAL
section of the corresponding libraries version map file.  The consensus
on review is that, in addition to mandating the addition of symbols to
the EXPERIMENTAL version in the map, we need a mechanism to enforce our
documented process of mandating that addition when they are introduced.
To that end, I am proposing this change.  It is an addition to the
checkpatches script, which scan incoming patches for additions and
removals of symbols to the map file, and warns the user appropriately.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>