support/dependencies/dependencies.sh: Remove some absolute paths

Buildroot fails to run on NixOS because it has no /bin/echo or
/bin/grep. Instead of relying on absolute paths, rely on tools to be
available in PATH. This should work for all systems.

Signed-off-by: Bjørn Forsman <bjorn.forsman@gmail.com>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>