docs: Set BOARD in quick build instructions
docs: Set BOARD in quick build instructions

Step 3 of the instructions in the "Building" section indicate to run a make
command that includes `make BOARD=${BOARD}`. This assumes that ${BOARD} is
set properly but the instructions to not specify to do so. The first
paragraph does link to documents that cover this but then go on to state
that you can safely ignore them.

Fix by adding a step to specify the BOARD env var, including a brief
description of where to find this value and a safe default.

BUG=b:219081299
BRANCH=none
TEST=view in gitiles

Signed-off-by: Bobby Casey <a3c107f0b4efaee07148ab0813ba992bba562a69@google.com>
Change-Id: I9b0a7f9b6b64ff2b1bb5844c5fa750514ecf6d1b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3460062
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