Move whitespace variables up to where all the other variables are

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Move up LOOKUP_PKG_RULE

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Increase $(BUILD_PKG) accuracy by checking build rules

This eliminates false-positive with packages with explicit empty build rules
like binutils. gcc is another false-positive, but it is another matter and is
harder to fix.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Add build matrix generation target

Based on a patch by Tobias Gruetzmacher <tobias-git@23.gs> (@TobiX).

Fixes #346.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>