raw.py: print a message when compressing the image

raw.py: print a message when compressing the image

Image compression may take very long time. The last message on the screen
before compression starts is "Generating map file(s)", and then a very long
delay, so that it looks like it takes so long to generate map files, but it is
not, it is actually the compression which is slow.

Let's inform users that we start compressing the image to make it obvious what
causes the delay.

Change-Id: Ie547451519aae7be5173c05bcfab1aca6c165148
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>