boards: esp32: add XIP support and enable bootloader build

Disable RTC WDT enabled (by default) by 2nd stage bootloader in ESP-IDF.
This WDT timer ensures correct hand-over and startup sequence from
bootloader to application.

Enabling bootloader caused system clock initialization to fail
when clock rate is greater then 80MHz. This also fixes
esp32 clock source code.

Signed-off-by: Mahavir Jain <mahavir@espressif.com>