AP_BLHeli,SRV_Channel: allow BLHeli support to be disabled

[muramura]

Summary

Allows HAVE_AP_BLHELI_SUPPORT to be overridden by board configuration, and uses it consistently in SRV_Channel.

This lets boards with serial RC output support disable BLHeli pass-through support when the board does not use BLHeli ESCs.

Classification & Testing

• Checked by a human programmer
• Non-functional change
• No-binary change
• Infrastructure change (e.g. unit tests, helper scripts)
• Automated test(s) verify changes (e.g. unit test, autotest)
• Tested manually, description below (e.g. SITL)
• Tested on hardware
• Logs attached
• Logs available on request

Built Copter for esp32s3m5stampfly with an extra hwdef setting:

define HAVE_AP_BLHELI_SUPPORT 0

The build completed successfully. I also checked the generated ELF and confirmed that no AP_BLHeli symbols were present.

Description

AP_BLHeli already had a HAVE_AP_BLHELI_SUPPORT define, but it was fixed to HAL_SUPPORT_RCOUT_SERIAL, and SRV_Channel still used HAL_SUPPORT_RCOUT_SERIAL directly for the BLHeli member, parameters, init, and telemetry update.

This change makes HAVE_AP_BLHELI_SUPPORT board-overridable and uses it consistently for BLHeli integration in SRV_Channel.