buildroot

manual: add section about dealing efficiently with big image files

As reported by Ryan, it is not well-known that most tools can deal
efficiently with big sparse files.

Add a section in the manual about this, with tar and cp used as
examples, and a hinting to the man pages for the others.

Reported-by: Ryan Barnett <rjbarnet@rockwellcollins.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Ryan Barnett <rjbarnet@rockwellcollins.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>