zephyr-flash-bossac.bbclass: Use internal bossac tool instead looking up PATH

Instead of looking in PATH on the host to find bossac we now depend on the
native variant we build and set the path to our yocto build tool.

Closes: https://git.ostc-eu.org/OSTC/planning/core-os/-/issues/240



Signed-off-by: Stefan Schmidt <stefan.schmidt@huawei.com>