[nrf] fix host mode build error
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
Merge pull request #92 from bgamari/master

spiflash: Add utility to convert capacity to bytes
rtc: Ensure next is initialized
rtc: Ensure time is valid before scheduling alarms
rtc: Set TAR before enabling interrupt
spiflash: Use enum values instead of magic numbers
spi: Add queued flag to spi_ctx_bare

This allows us to ensure that the queue never forms a cycle, e.g. when
the same context is used for multiple transactions. These issues are
very difficult to debug and therefore the small cost of this measure
should be well worthwhile.