Add configure --with-sysconfig-leaf-dir=SUBDIR to set CONFIG_LEAF_DIR

Add configure --with-sysconfig-leaf-dir=SUBDIR to set CONFIG_LEAF_DIR

Set CONFIG_LEAF_DIR with configure to give control if needed. The
check for the correct value if the option is not specified is tricky.
Since other packages (such as grub2) started to populate also
/etc/default/ a given system may have both directories.
Use "default" only if /etc/sysconfig does not exist. "sysconfig"
remains the default.

Move the variable from StdGNU.mk to Linux.mk because thats the only
place where it is used.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
[ ijc -- ran autogen.sh as requested ]