bmaptool: make warnings and errors more visible

bmaptool: make warnings and errors more visible

People usually do not read docs and forget to use the --bmap option, and then
wonder why flashing is not fast enough. bmaptool prints some kind of warning
in this case, but it is not visible enough. Make it to be more visible:

1. Change the message to say that flashing will be slow.
2. Make the warning/error prefix to use capital letters.
3. Make the output to be coloured.

This should hopefully draw more attention and people will notice warnings
and errors more easily.

Change-Id: Iccf44430c5b9e671a5d540c2966fead3aa800337
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>