Reformat extlibs SConscripts to be more Pythonic

Reformat extlibs SConscripts to be more Pythonic

There are not intended to be any functional changes.

Minor additonal changes:
  starred error/notice boxes trimmed to not be so wide, drop trailing stars
  minor language cleanup in a few messages
  a little reorg of comments in mbedtls
  instead of "print MESSAGE; Exit(1)" now do "msg = MESSAGE; Exit(msg)"
    to send error messages to the error stream instead of the output stream

Bug: https://jira.iotivity.org/browse/IOT-1745
Change-Id: I9d7da2559eee30c8a3e076aeeae7b5a5f2c40843
Signed-off-by: Mats Wichmann <mats@linux.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/21199
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Phil Coval <philippe.coval@osg.samsung.com>