cookbook: add 'rpm' package to build it with CPack.

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>
cookbook: add 'devscripts' package to recipe.

Even though, this package is not a direct build dependency, it provides
useful check scripts such as 'lintian' and 'checkbashisms'

Signed-off-by: Emmanuel Roullit <emmanuel.roullit@gmail.com>