Add external linkage to start entry point function

[gustavonihei]

This PR intends to extern the symbol for start entry point function, which is necessary for restoring the ESP32-C3 build for MCUboot bootloader (feature = mcu-boot).

Patches here are similar to those previously applied on esp-rs/esp-hal#49 for ESP32-C3 target.

[bjoernQ]

I think the function should be at least a naked function since otherwise the compiler will generate the function prologue and epilogue

Maybe I miss something but I also think it's not really necessary - will comment on that esp-hal PR

[gustavonihei]

I think the function should be at least a naked function since otherwise the compiler will generate the function prologue and epilogue

The compiler won't generate the epilogue for a function that does not return. I checked the assembly and the prologue is also elided, maybe due to compiler inter-procedural optimizations.

Disassembly of section .rwtext:

40380000 <_start>:
40380000:       4081                    li      ra,0
40380002:       4101                    li      sp,0
40380004:       4181                    li      gp,0
40380006:       4201                    li      tp,0
40380008:       4281                    li      t0,0
...

[gustavonihei]

Closing PR in favor of a simpler alternative solution suggested by @bjoernQ in esp-rs/esp-hal#417 (comment).

[bjoernQ]

I think the function should be at least a naked function since otherwise the compiler will generate the function prologue and epilogue

The compiler won't generate the epilogue for a function that does not return. I checked the assembly and the prologue is also elided, maybe due to compiler inter-procedural optimizations.

Disassembly of section .rwtext:

40380000 <_start>:
40380000:       4081                    li      ra,0
40380002:       4101                    li      sp,0
40380004:       4181                    li      gp,0
40380006:       4201                    li      tp,0
40380008:       4281                    li      t0,0
...

Oh, that's interesting!