Updates to GCC warning level flags

In gcc4mbed, I have been running with "-Wall -Wextra" and then
disabling a couple of noisy warnings that result.  In particular, I
disable the unused-parameter and missing-field-initializers warnings.
The first commonly goes off for implementation of virtual methods or
other overridable functions where not all parameters are required for
every override.  I don't find the second warning to be all that useful
anyway since missing structure field initializers will be set to 0
according to the C language specification.  The RTOS code uses this
language feature and I see no reason that it shouldn't :)