In some cases, when NuttX configuration changes and this makes the
object list used to build one of the .a libraries change as well,
since the command used to build it is "ar crs" and this simply appends
the list of object files, the library could still include object
files from prior builds. This commit modifies the ARCHIVE macro to
erase the .a file if it already exists.
Since in some cases this behavior was actually expected (object
files from a subdirectory were appended to a library created one
level above) I added a ARCHIVE_ADD which works as ARCHIVE did.
This change should greatly improve behavior of building after
configuration changes.
Adam Feuer