support/download: svn non-interactive in BR2_SVN

Instead of overriding the _svn command and injecting --non-interactive,
change the default value of BR2_SVN to include this flag so the end user
can choose not to use the flag.

This change helps users behind corporate system rules which may not
allow them to locally cache credentials and require interactive mode.

Signed-off-by: Sam Voss <sam.voss@rockwellcollins.com>

[Originally implemented by]
CC: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>