microkernel: Don't initialize packets in a command packet set

Reworks the internal design of a command packet set so that
the command packet array is split out into a separate variable
lying in uninitialized memory. This shrinks the command packet's
data section footprint to almost nothing.

Note: A side effect of this change is that it is no longer possible
to define a command packet set as a "static" variable since the
CMD_PKT_SET_INSTANCE() macro now generates two variables.

Change-Id: I9c7ebe637edf879758589ff4a26ace1303790bf7
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>