Rule for system/build.prop is also from macro

Like other <partition>/build.prop files, the build rule for
system/build.prop is also created using the build-properties macro.

system/build.prop has a few number of auto-generate properties that the
macro doesn't create. Those properties are emitted to intermediate files
using manually crafted rules, the paths to the files are fed to the
macro, where their contents are copied to system/build.prop.

This change also refactors the code fragment that defines BUILD_*
variables that identifes the given build.

Bug: 117892318
Test: m

Change-Id: Ic218283b40fedeaddacb3fda93167edbec86094c