So that they're set properly during dumpvars / product loading.
SHELL is already set in common/core.mk, so it's not necessary to be in
core/main.mk as well (which quickly loads core/config.mk, which loads
common/core.mk).
Test: get_build_var TOP
Test: get_build_var TOPDIR
Change-Id: I8179ac32628b74ccf29851dc53bf83cfb4b280fd
In preparation for setting up another Kati stage, move some common
settings and tools into a common folder. This way it's a bit easier to
see that they're safe to use, and that they shouldn't depend on anything
outside of the common folder.
Bug: 117463001
Test: build-aosp_arm.ninja is the same before and after
Change-Id: Ief4b75a4dbe45b73ffd03bf32c60695c816d979d
