support/graph-size: add option to report size with IEC prefixes

When dealing with embedded devices, storage is more often than not some
kind of flash device, on which the memory is usually counted as powers
of 1024 instead of powers of 1000. As such, people may prefer reports
using IEC prefixes [0] instead of the SI prefixes.

Add an option to that effect.

We use argparse's ability to use custom actions [1] [2], to provide a
set of options that act on a boolean, but has a single help entry and
internally ensures consistency of the settings. We could have been using
the more conventional store_true/store_false actions instead, but that
would have meant either two help entries, one for each set of options,
and/or some logic after parse_args() to check the validity of the
settings.

[0] https://en.wikipedia.org/wiki/Binary_prefix
[1] https://docs.python.org/2/library/argparse.html#action
[2] https://docs.python.org/2/library/argparse.html#argparse.Action

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

docs/manual/common-usage.txt

@@ -326,6 +326,9 @@ to further control the generated graph. Accepted options are:
   contributing less than 1% are grouped under _Others_. Accepted values
   are in the range `[0.0..1.0]`.
 
+* `--iec`, `--binary`, `--si`, `--decimal`, to use IEC (binary, powers
+  of 1024) or SI (decimal, powers of 1000; the default) prefixes.
+
 .Note
 The collected filesystem size data is only meaningful after a complete
 clean rebuild. Be sure to run +make clean all+ before using +make