package/ntp: override host shell check

package/ntp: override host shell check

Add a patch from the upstream AutoGen package that allows POSIX_SHELL
to be taken from the environment, then define that to be '/bin/sh'.

Since we are cross-compiling, the original behaviour of detecting the
host shell is not useful as we cannot assume that the target uses the
same shell, and it can prevent builds being reproducible because a
different host environment will result in a different target binary.

Signed-off-by: James Byrne <james.byrne@origamienergy.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>