Fix compile warning in cpu.cpp

Fix
cpu/cpu.cpp: In function ‘void report_display_cpu_cstates()’:
cpu/cpu.cpp:616:28: warning: ‘_core’ may be used uninitialized in this function [-Wmaybe-uninitialized]
cpu/cpu.cpp: In function ‘void report_display_cpu_pstates()’:
cpu/cpu.cpp:797:27: warning: ‘_core’ may be used uninitialized in this function [-Wmaybe-uninitialized]
RAPL: Use Powercap sysfs

Linux Powercap sysfs with Intel RAPL driver exists from 3.13.rc1.
Use this to estimate RAPL domain power instead of MSRs. Since
these MSRs are non-architectural, they can be changed from
processor model to model. But by using powercap sysfs, kernel
driver will take care about processor specific variations.
Use package name to identify domains

Here the cpu is identifying a package id. Currently the RAPL
package name is not used, but by changing to powercap sysfs
package name, it can be used to identify RAPL package domains
in a multi processor system.
Update README with configs

Update powercap sysfs and Intel RAPL driver config entries as
suggested configs.