luci-base: revert removal of HOST_BUILD_DIR definition

PR #3352 removed the luasrcdiet subpackage definition from
luci-base Makefile, and it also removed the HOST_BUILD_DIR
definition, which looked like it was related to only luasrcdiet,
but apparently that HOST_BUILD_DIR is needed also for the main
luci-base package.

Fixes #3433

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>