package/pkg-utils.mk: rationalise kconfig option mangling

package/pkg-utils.mk: rationalise kconfig option mangling

Currently, we have three macros that may mangle a .config file. All
three are modeled after the same pattern: removing the existing option
from the .config file, then adding the new definition for that option;
all three also implement that pattern with the same commands: sed and
echo.

This is all good so far, because it was simple enough, and they always
worked on a file passed in parameter.

However, we're soon going to change this file parameter to make it
optional, so that the file will then be auto-deduced for the current
package. In that case, the file to sed and echo into will be a more
complex structure than just the parameter.

As such, move the actual mangling down to a helper macro, that is called
from the three existing ones.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>