usb/cdc: do not crash if no data ready cb is present

Noticed-by: mkad
Add pin change interrupt infrastructure

Each PORT peripheral has a single interrupt which can be used to detect
changes on any of that peripheral's pins. Because of this, it is
necessary to multiplex pin change interrupt handling. This patch adds
the infrastructure for this.
nrf: Port to pin change infrastructure
Split up linker script
Merge pull request #99 from bgamari/pin-change

Common pin change interrupt handling
Move target sections to directly after .text
Merge branch 'master' of https://github.com/mchck/mchck
Merge pull request #102 from bgamari/pin-change

Move target sections to directly after .text



This fixes overlapping section errors during linking.
Merge branch 'master' of https://github.com/mchck/mchck
lib/pin: allow macros as arguments to PIN_DEFINE_CALLBACK

Noticed-by: @smeding
toolchain/ld: do not pack .pin_hooks.* sections
Merge branch 'master' of https://github.com/mchck/mchck