[Buildroot] new <pkg>_CONFIG_FIXUP variable

[Stefan Fröberg]

This patch provides new variable called <pkg>_CONFIG_FIXUP.
It's purpose is to fix wrong prefixes of $(STAGING_DIR)/usr/bin/*-config file(s)
that many library packages install after staging installation phase.

Some packages provide multible *-config files.

So as suggested by Thomas Petazzoni and Gustavo Zacarias this 
variable can be feeded multible files.

All filenames are relative to $(STAGING_DIR)/usr/bin

Example:

DIVINE_CONFIG_FIXUP = divine-config

or:

IMAGEMAGICK_CONFIG_FIXUP = Magick-config Wand-config

Regards
Stefan