docs/manual/adding-packages-directory.txt: reorder select/depends

From [1]:
"Even though the ordering has absolutely no consequences in Kconfig, it
 is not logical (when reading). It is more logical and far easier to
 understand when depends come first, followed by the selects."

 Also, the Config.in example in the manual suggests to use this coding
 style [2]."

Use the correct coding style in the chapter "Choosing depends on or select"
in the manual.

[1] http://lists.busybox.net/pipermail/buildroot/2015-October/142955.html
[2] https://nightly.buildroot.org/manual.html#_coding_style



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>