Asterisk: Add 'flash_spec' UCI config to support multiple flash-hook handling for different specs

"uk"   - the default config which support flash-hook (R) only triggering call waiting and 3-way conference.
         R4 and R5 are for attended and unattended call transfer respectively with a timer.
"etsi" - Using R0, R1, R2, to trigger different ways of handling call waiting. R3 for 3-way conference.
         R4 and R5 are for attended and unattended call transfer respectively without a timer.