AP_Relay: add hwdef-level RELAYn_DEFAULT compiled default for boot state

[Hwurzburg]

RELAYn_PIN_DEFAULT already lets a board's hwdef.dat set the relay pin at compile time, but the relay's ON/OFF boot state had no equivalent - it was hardcoded to DefaultState::OFF, with the only override being a RELAYn_DEFAULT parm line in defaults.parm. Several board hwdefs (and open board-support PRs) assumed a hwdef-level default already existed and either documented a boot state defaults.parm never delivered, or carried an already-written but silently dead RELAYn_DEFAULT hwdef define.

Add optional RELAY1_DEFAULT..RELAY6_DEFAULT macros, following the same pattern as the existing pin defaults, applied in set_defaults() before init() configures the GPIO. Verified in SITL with --define=RELAY2_DEFAULT=1 that the compiled default reaches the live parameter (RELAY2_DEFAULT reads back as 1 with a fresh eeprom, RELAY1_DEFAULT remains the hardcoded 0 fallback when unspecified).

Summary

This allows Relays setup in hwdef to actually force the VTX power, etc on after boot...Currently , if not added in defaults.parm, the VTX will revert to OFF after boot in those cases where its desired that it be on wihtout user intervention.

Classification & Testing (check all that apply and add your own)

• 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

[andyp1per]

LGTM