Config.in: update description of BR2_PREFER_STATIC_LIB

With time BR2_PREFER_STATIC_LIB option meaning changed from "prefer static
libs when possible" to "do not build dynamic libs and build statically linked
applications".

This patch changes the option description.

Note that variable name is kept unchanged on purpose - it will be modified later
when real "prefer static" mode will be introduced.

[Peter: tweak help text as suggested by Thomas and Yann]
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>

Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Cc: Peter Korsgaard <peter@korsgaard.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>