Turn the static lib option into a choice with more options

This commit turns the single static option into a choice, which offers
various possibilities:

 1. Build and use static libraries only;
 2. Build both shared and static libraries, but use shared libraries;
 3. Build and use shared libraries only.

On most platforms, (2) is currently the default, and kept as the
default in this commit. Of course, on certain platforms (Blackfin,
m68k), only option (1) will be available.

In addition to the introduction of the Config.in options, this commit
also:

 * Removes the 'select BR2_STATIC_LIBS' from 'BR2_BINFMT_FLAT', since
   with the use of a choice, we are guaranteed that BR2_STATIC_LIBS
   will be selected when the binary format is BR2_BINFMT_FLAT, since
   BR2_STATIC_LIBS will be the only possible solution in the choice.

 * Changes package/Makefile.in to use the proper
   --{enable,disable}-{shared,static} options for autotools packages.

[Thomas: remove useless empty newline right after 'choice'. Noticed by
Yann E. Morin.]

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>