configs/*: add stress-ng and demidecode utility to buildroot

Add stress-ng and dmidecode utility into buildroot. The stress-ng
utility is a workload generator tool, and can be used to wake the
desired number of CPUs from idle state. The dmidecode is a linux utility
to get the system information from the SMBIOS tables.

Change-Id: I9aae77080d178d844035a17bfb40457bc23719b9
Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>