openssl: fix bashism in c_rehash shell script

openssl: fix bashism in c_rehash shell script

This script claims to be a /bin/sh script, but it uses
a bashism:

from checkbashisms:

possible bashism in meta/recipes-connectivity/openssl/openssl/openssl-c_rehash.sh line 151 (should be 'b = a'):
	    if [ "x/" == "x$( echo ${FILE} | cut -c1 -)" ]

This causes build issues on systems that don't have
/bin/sh symlinked to bash:

Updating certificates in ${WORKDIR}/rootfs/etc/ssl/certs...
<builddir>/tmp/sysroots/x86_64-linux/usr/bin/c_rehash: 151: [: x/: unexpected operator
 ...

Fix this by using POSIX shell syntax for the comparison.

Signed-off-by: André Draszik <adraszik@tycoint.com>
Reviewed-by: Sylvain Lemieux <slemieux@tycoint.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>