util: Add --os support to clangd_config.py
util: Add --os support to clangd_config.py

BRANCH=none
BUG=b:236389226,b:176500425
TEST=./util/clangd_config.py bloonchipper
TEST=./util/clangd_config.py --os auto bloonchipper
TEST=./util/clangd_config.py --os ec bloonchipper
TEST=./util/clangd_config.py --os zephyr bloonchipper
     # Expect error
TEST=./util/clangd_config.py herobrine
     # Expect error

Signed-off-by: Craig Hesling <383db090fd8ef907a606ac8a9eccd2962bc489cf@chromium.org>
Change-Id: Iee6c7d8626f046f4950bce48f5a5be0b84f4eeeb
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3931851
Reviewed-by: Andrea Grandi <f017805e76e0e7a74072a3bac1b6e29e65eedeab@google.com>

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