feature: show defconfig content

[fdcavalcanti]

Adds get_defconfig method to Board class and CLI command to get the content of a defconfig, allowing a quick read without having to navigate the source code.

Basically, do:

$ ntxbuild list defconfigs esp32s3-devkit --read usbmsc

And it prints the content of this defconfig:

Defconfig content for usbmsc:
#include "../usb_device/defconfig"
CONFIG_BCH=y
CONFIG_DEV_ZERO=y
CONFIG_ESP32S3_SPIFLASH=y
CONFIG_ESP32S3_SPIFLASH_MTD_BLKSIZE=512
CONFIG_SYSTEM_USBMSC=y
CONFIG_SYSTEM_USBMSC_DEVMINOR1=0
CONFIG_SYSTEM_USBMSC_DEVMINOR2=1
CONFIG_SYSTEM_USBMSC_DEVPATH1="/dev/ram10"
CONFIG_SYSTEM_USBMSC_DEVPATH2="/dev/esp32s3flash"
CONFIG_USBMSC=y
CONFIG_USBMSC_EPBULKIN=2
CONFIG_USBMSC_EPBULKOUT=1