Accept TARGET_DEVICE_DIR preserved from previous product load

During a build, we load product configuration three different times --
to dump some initial variables, when parsing the CleanSpec.mk files, and
again when we parse the Android.mk files.

The BoardConfig.mk find commands use -path, which isn't supported by the
Kati find optimizer, so we end up spending ~125ms in each of the above
cases (on my machine, internal master -- aosp is closer to 30ms).

Instead, do the search during the initial dumping of variables, and pass
in TARGET_DEVICE_DIR into the later runs, using that to load the
BoardConfig.mk file.

Bug: 78020936
Test: out/build-taimen.ninja is identical
Test: out/soong.log shows that we're not running these finds again
Change-Id: I6f186e1879aa362528b48b8a00be3f7a9f88bfc5