Add busy detection to chan_mobile

From the patch author:

"First this patch adds general support for busy detection. It also adds support
 for the ECAM command at Sony Ericsson phones and also signals busy when only
 early media was received but the call got not answered."

Review: https://reviewboard.asterisk.org/r/323

(closes issue ASTERISK-14527)
Reported by: Artem Makhutov
Tested by: Artem Makhutov
patches:
  busy-full5.patch uploaded by artem (license 5757)



git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@379144 f38db490-d61c-443f-a65b-d21fe96a405b