docs: Update "Getting Started Quickly"
docs: Update "Getting Started Quickly"

Instructions in this document have become out of date.

Align the content with the "Chromium OS Developer Guide":
- Add missing package to fix local build error with libusb header
- Use ~/chromiumos folder for chroot to be consistent with dev guide
- Set missing env variable $BOARD
- Replace deprecated script ./build_pacakges
- Remove minilayout and use stable branch to make the build more
  reliable

BRANCH=none
BUG=b:219082979
TEST=View of Markdown preview in VSCode

Signed-off-by: Andrea Grandi <f017805e76e0e7a74072a3bac1b6e29e65eedeab@google.com>
Change-Id: I3fdac3b346098575a0957394e80052626ecbb551
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3466428
Reviewed-by: Bobby Casey <a3c107f0b4efaee07148ab0813ba992bba562a69@google.com>
Reviewed-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec