package/pkg-utils.mk: kconfig mangling defaults to current package's .config

package/pkg-utils.mk: kconfig mangling defaults to current package's .config

The kconfig mangling macros currently operate on the caller-supplied
.config file, on the assumption that the caller will always know what
file to mangle.

This was correct so far, as packages would indeed only mangle their
own .config files.

However, the Linux kernel does its mangling based on whether some
other packages are enabled or not. That list of conditional mangling
is getting bigger and bigger with each new package that needs such
mangling, culminating with the pending firewalld one [0]. Furthermore,
this mangling is not accessible to packages in br2-external trees. So
we'll want to have packages provide the mangling commands.

So we'll want the mangling to be done on the Linux' .config file in
the expanding package context, not in the package calling the macros.

But packages do not, and should not have knowledge about where the
.config file is, nor how it is named.

So we make the parameter to specify the .config file to mangle
optional.  If it is set, this is what the macros will mangle; if it is
not set, the expanding package's .config file will be used.

This has the added benefit that we do not have to repeat in the
expanding package context the knowledge of how the .config file is
named:

    FOO_KCONFIG_DOTCONFIG = .config
    define FOO_KCONFIG_FIXUPS_CMDS
        $(call KCONFIG_ENABLE_OPT,BLA)
    endef

[0] http://lists.busybox.net/pipermail/buildroot/2020-March/278683.html

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>