4.2-rc2

This is the second 4.2 release candidate. Change log since rc1:

• Added continue and set to the gdb cheat sheet
• Fixed an error when trying to debug apps without a .bss section
• Added support for debugging workers (requires SDK 3.10-beta6)
• Removed a spurious use of the word 'echo' from the gdb preamble.
• Worked around a bug in Apple's ancient version of libiconv.
• Fixed a bug when passing extra arguments to waf through pebble build.
• Directed update notices to stderr instead of stdout.