buildroot

docs/manual: add check-package to "Tips and tricks"

And add two references to it: in "Submitting patches" and in "Adding new
packages to Buildroot" sections.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>