soc: esp32: Define __start as a C function

The first stage bootloader, part of the ESP32 ROM, already sets up
a stack that's sufficient to execute C programs.  So, instead of
implementing __stack() in assembly, do it in C to simplify things
slightly.

This ESP32-specific initialization will perform the following:

  - Disable the watchdog timer that's enabled by the bootloader
  - Move exception handlers to IRAM
  - Disable normal interrupts
  - Disable the second CPU
  - Zero out the BSS segment

Things that might be performed in the future include setting up the
CPU frequency, memory protection regions, and enabling the flash
cache.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>