doc: make build process quiet

make build process less verbose and enable 'make htmldocs' from the top
tree without a need for ZEPHYR_GCC_VARIANT to be defined.

Change-Id: I385667e3d240205913fa806b7481d5ed4e83fbc4
Signed-off-by: Anas Nashif <anas.nashif@intel.com>
2 files changed