build: include BUILD_VARIANT in PKG_BUILD_DIR

This changes the default PKG_BUILD_DIR to take BUILD_VARIANT into
account (if set), so that packages do not need to manually override
PKG_BUILD_DIR just to handle variants.

This also updates most base packages with variants to use the updated
default PKG_BUILD_DIR.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>
(cherry picked from commit e545fac8)