Squashed commit of the following:

apps/builtin/exec_builtin.c:  Try posix_spawn if builtin apps do not have have an entry point.
    apps/Application.mk: introduce MODULE config to simplify tristate(m)
    apps/nsh:  Change the nuttx shell module type to tristate
    apps:  Add loadable application support
    script/mksymtab:  Generate symbol table name by default
    apps/builtin:  Allow loadable applications can register with apps/builtin.

examples/random/random_main.c

@@ -77,7 +77,7 @@
  * rand_main
  ****************************************************************************/
 
-#ifdef CONFIG_BUILD_KERNEL
+#ifdef CONFIG_BUILD_LOADABLE
 int main(int argc, FAR char *argv[])
 #else
 int rand_main(int argc, char *argv[])