toolchain/mk: add rule for linkdep cache objects

The previous linkdep cache code worked only because of the mere
coincidence that toolchain/cache and examples/* both have a ../../
distance to the repository root.  Fix this by adding a rule for linkdep
cache object files.  This allows building out-of-tree projects.

Noticed-by: Roy Smeding
spiflash: Add utility to convert capacity to bytes
Merge pull request #92 from bgamari/master

spiflash: Add utility to convert capacity to bytes