flex: change config text of 'target binary' option

The buildroot manual contains a list of deprecated items, with their config
title text, and the location in the config menu. If the config text does not
mention a package name, this can be confusing.

For example, the symbol BR2_PACKAGE_FLEX_BINARY has as text:
'Install tool in the target', which outside of the flex context makes no
sense at all.

To make sure the deprecated packages list in the manual is understandable,
rename the (deprecated) flex 'target binary' config option.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>