Introduce a prelude to mcu-board-support to make esp-hal available when the entry point comes from esp_hal::main

esp_hal::main's code requires the availability of `esp_hal` as import.

docs/astro/src/content/docs/guide/platforms/embedded.mdx

@@ -99,6 +99,8 @@ The main.rs will look something like this
 #![cfg_attr(not(feature = "simulator"), no_main)]
 slint::include_modules!();
 
+use mcu_board_support::prelude::*;
+
 #[mcu_board_support::entry]
 fn main() -> ! {
     mcu_board_support::init();