buildroot

linux: clarify help text for 'custom tarball'

The help text for Linux option 'Custom tarball' only refers to ftp or
http tarballs, while in reality file or scp protocols are also
supported.

Triggered by a recent support question, update the help text to clarify
this.

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