Prevent false positive during check source

At runtime CFLAG _FORTIFY_SOURCE can provide false positive during its
checking. Change _FORTIFY_SOURCE level to 1 to avoid wrong comportement
of the package.

Change-Id: Ice4b386c253c87903e58f82a446416831cb4c6bf
Signed-off-by: Frederic Marec <frederic.marec@iot.bzh>