build: Introduce a variable "$sudo" and honour WIRINGPI_SUDO
We are going to want to not run sudo sometimes. So replace literal
invocations of sudo with a variable, which is set to `sudo' by
default, but can be overridden from the environment.
No functional change if the environment variable is not set.
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>