makefiles: improve build order

makefiles: improve build order

Separates various groups of files to be built in logical succession.
In each layer, the core module (target.c, nand.c, etc.) is built _after_
their helper modules (e.g. image.c, nand_ecc.c) but _before_ any of
their drivers (e.g. arm966e.c, mx3_nand.c).

This allows problems introduced at the bottom of the stack to result
in build failures as soon as possible, as the helpers and core should
wrap portions of them.