skeleton: use BR2_SYSTEM_DEFAULT_PATH as default PATH

skeleton: use BR2_SYSTEM_DEFAULT_PATH as default PATH

We substitute the path specified in system/skeleton/etc/profile with
the path specified in the configuration variable
$(BR2_SYSTEM_DEFAULT_PATH).

$(BR2_SYSTEM_DEFAULT_PATH) is a Kconfig string, so it is already
double quoted. This means that export PATH=value will now be export
PATH="value" in /etc/profile, which is perfectly fine.

Signed-off-by: Markus Mayer <mmayer@broadcom.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Thomas: rework commit log about the double quoting]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>