buildtools-tarball: fix unbound variable issues under 'set -u'

buildtools-tarball: fix unbound variable issues under 'set -u'

When Bash runs with 'set -u' (nounset), accessing an unset variable
directly (e.g. [ -z "$SSL_CERT_FILE" ]) causes a fatal "unbound variable"
error. As a result, the fallback logic to set SSL_CERT_FILE/SSL_CERT_DIR
is never triggered and the script aborts.

The current code assumes these variables may be unset or empty, but does
not guard against 'set -u'. This breaks builds in stricter shell
environments or when users explicitly enable 'set -u'.

Fix this by using parameter expansion with a default value, e.g.
"${SSL_CERT_FILE:-}", so that unset variables are treated as empty
strings. This preserves the intended logic (respect host env first, then
CAFILE/CAPATH, then buildtools defaults) and makes the script robust
under 'set -u'.

Signed-off-by: Haixiao Yan <haixiao.yan.cn@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>