rp-pppoe: pass the appropriate path for various tools

rp-pppoe: pass the appropriate path for various tools

The rp-pppoe configure script uses AC_PATH_PROG() to find the path of
a number of tools on the build machine. But in fact, those paths are
not used to call the tools on the build machine, but rather to replace
variables in scripts that are executed in the target.

Due to this, if those tools on the build machine are not located at
the same place as their equivalent in the target, the scripts will
fail to run. To address this, we pass the necessary cache variables to
override the autoconf checks.

The path from pppd has been taken from the build result of the pppd
package. The path for setsid, id and echo have been taken from the
build result of Busybox, assuming that full-blown variants of those
tools would be installed at the same location.

Fixes bug #8811.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>