legacy: update the BR2_PREFER_STATIC_LIB option

Since the refactoring of the type of libraries in the target filesystem,
these options belong to a choice, and the default is "shared only".

The former option BR2_PREFER_STATIC_LIB has been moved in the legacy
menu, and only selects the BR2_STATIC_LIBS ("static only") option.
However, because BR2_STATIC_LIBS belongs to a choice, when
BR2_PREFER_STATIC_LIB is selected, it does not affect the choice.
So, existing Buildroot configurations, which have BR2_PREFER_STATIC_LIB
set, will silently end with BR2_SHARED_LIBS ("shared only") enabled.

So, the backward compatibility is not kept.

Besides, the legacy BR2_PREFER_STATIC_LIB option does not select
BR2_LEGACY, so the user is not notified of this legacy option, and the
build silently continues with some undesired changes.

This patch removes the inefficient BR2_STATIC_LIBS selection, correctly
selects the BR2_LEGACY option and extend the help message so the user
can easily know how to update his/her Buildroot configuration.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>