buildroot

scripts: fix for POSIX compliance

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

Of the three affected scripts, two are explicitly called vi #!/bin/bash.
Those two do not _need_ the fix, but gets it nonetheless, in case we
later switch to a POSIX-compliant shell for those scripts.

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