NuttX: fixes for NuttX build

In the upstream tree ringbuffer.h includes the method implementations
in the header file which causes multiple definitions in the link for
other targets. Changed so ringbuffer.cpp is build separately for other
platforms and is included by ringbuffer.h on NuttX.

uORB changes do not link without uORBTest_UnitTest.cpp enabled for
the NuttX build.

px4_getopt was not exported and wasn't visible in NuttX build.

The makefiles were restored to be as close as possible to upstream
so the NuttX build builtin's work again. The code will have to be
refactored after the merge.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/platforms/common/px4_getopt.c

@@ -121,7 +121,7 @@ static int reorder(int argc, char **argv, const char *options)
 // Argv is changed to put all options and option args at the beginning,
 // followed by non-options.
 //
-int px4_getopt(int argc, char *argv[], const char *options, int *myoptind, const char **myoptarg)
+__EXPORT int px4_getopt(int argc, char *argv[], const char *options, int *myoptind, const char **myoptarg)
 {
 	char *p;
 	char c;
@@ -149,4 +149,3 @@ int px4_getopt(int argc, char *argv[], const char *options, int *myoptind, const
 	}
 	return -1;
 }
-