rename packages to buildroot_packages

fix BR2_SAMPLE_PACKAGE to PACKAGE_SAMPLE_PACKAGE on README

Add some more README.adoc to subdirs.

buildroot_packages/sample_package/sample_package.c

@@ -0,0 +1,7 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+int main(void) {
+	puts("hello sample_package");
+	return EXIT_SUCCESS;
+}