manual/user guide/customization: change recommendation for package paths

The Buildroot manual was recommending following paths for project-specific
packages:
    package/<company>/<boardname>/foo/
    $BR2_EXTERNAL/package/<boardname>/foo/

However, if a company has several boards, it is often the case that some
packages are common for different boards. Therefore, introducing a
<boardname> path component is not ideal.

This patch changes the recommendation to:
    package/<company>/foo/
    $BR2_EXTERNAL/package/foo/

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