docs/manual: rephrase part about rebuilding packages

There is no need to tell people that they should remove stamp files: they
should use the make <pkg>-reconfigure and make <pkg>-rebuild make targets
instead.  We still keep an explanation about stamp files, just to give the
user an insight on how Buildroot works internally.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>