Move TOP/TOPDIR to build/make/common/core.mk

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