Fix configure for s390 qemu on alpine and other busybox environments

Fix configure for s390 qemu on alpine and other busybox environments

In alpine docker image the qemu-system-s390x build is broken and
it throws this error:
qemu-system-s390x: Initialization of device s390-ipl failed: could not
load bootloader 's390-ccw.img'

The grep command of busybox uses regex. This fails on binary data
(e.g. stops on every \0), so it does not identify the string
BiGeNdIaN in the test case big/little. Therefore, it assumes
that the architecture is little endian.

This fix solves the grep problem by printing the content of
TMPO with strings

Signed-off-by: Alice Frosi <alice@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
[some changes to patch description, add -a option to strings]
Message-Id: <20180130133828.77336-2-borntraeger@de.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>