build: override hardcoded paths to bison and m4 to make the SDK more relocatable

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

Backport of r42300

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42301 3c298f89-4303-0410-b956-a3cf2f4a3e73