manual/getting started: minor improvements to 'Using Buildroot' section

This patch renames the section 'Using Buildroot' into 'Buildroot quick
start' as real details about Buildroot usage will be in the User Guide
rather than in 'Getting Started'.

Additionally, add a cross-reference to the section about configuring
Buildroot.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>