skeleton: fix atstk1005, ststk100x and atngw100 for POSIX compliance

Using two '=' for string comparison is a bashism.
Revert to using one, as stated in POSIX 1003.1-2008.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>