manual: restructure 'Adding packages' chapter

The depends-on-vs-select part of the manual really deserves its own
section title (especially because it is referred to and the xref gets
a 'sinpara' in PDF if the section doesn't have a title). So restructure
the surrounding sections to reduce the section nesting depth.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

docs/manual/adding-packages-directory.txt

@@ -13,7 +13,7 @@ New subdirectories are discouraged, however.
 
 
 +Config.in+ file
-^^^^^^^^^^^^^^^^
+~~~~~~~~~~~~~~~~
 
 Then, create a file named +Config.in+. This file will contain the
 option descriptions related to our +libfoo+ software that will be used
@@ -51,6 +51,9 @@ source "package/libfoo/Config.in"
 --------------------------
 
 [[depends-on-vs-select]]
+Choosing +depends on+ or +select+
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 The +Config.in+ file of your package must also ensure that
 dependencies are enabled. Typically, Buildroot uses the following
 rules:
@@ -164,7 +167,7 @@ Further formatting details: see xref:writing-rules-config-in[the
 coding style].
 
 The +.mk+ file
-^^^^^^^^^^^^^^
+~~~~~~~~~~~~~~
 
 Finally, here's the hardest part. Create a file named +libfoo.mk+. It
 describes how the package should be downloaded, configured, built,