Fix UEFI mode detection

If '/sys/firmware/efi' it means the kernel has booted in EFI mode (https://wiki.archlinux.org/index.php/UEFI#Detecting_UEFI_Firmware_bitness).
New efivarfs interface replace the old sysfs-efivars (https://wiki.archlinux.org/index.php/UEFI#UEFI_Variables_Support_in_Linux_Kernel).
fix GRUB2 installation in UEFI system
Merge pull request #101 from simone91/master

Fixed some problems on UEFI systems