flex: fix build for nommu systems

The flex binary uses fork() so it breaks on !MMU builds.
Since we usually don't require flex in the target and the common
scenario is that we just want libfl in staging reverse the options so
that BR2_PACKAGE_FLEX just builds and install libfl.a and change the
LIBFL option to BR2_PACKAGE_FLEX_BINARY to install the binary in the
target.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>