This change introduces a new subtree in the library's directory structure.
Platform-specific code now should be placed into the corresponding directory
in src/platform/. To illustrate this, prng.h has been split into three
distinct files, one for each platform (contiki, lwip, posix) and moved into
the respective subdirectory.
Commit 1baa816 moved the implementation of coap_clock_init() and
coap_ticks_to_rt() to coap_time.c. This commit updates the public
API of the shared lib accordingly.
Olaf Bergmann