Fix .gitignore to avoid ignoring prebuilt libraries.

Also, generally clean-up the .gitignores and farm off separate versions for the NuttX/Apps directories to keep things tidy.

nuttx/.gitignore

@@ -0,0 +1,28 @@
+*.a
+.config
+.config-e
+.configX-e
+.depend
+.version
+arch/arm/include/board
+arch/arm/include/chip
+arch/arm/src/board
+arch/arm/src/chip
+include/apps
+include/arch
+include/math.h
+include/nuttx/config.h
+include/nuttx/version.h
+Make.defs
+Make.dep
+mkdeps
+nuttx
+nuttx-export.zip
+nuttx.bin
+nuttx.hex
+setenv.sh
+System.map
+tools/mkconfig
+tools/mkconfig.exe
+tools/mkversion
+tools/mkversion.exe