Explicitly supply the ACPI RSDP address to kexec on efi systems

Kexec doesn't setup an efi boot and on some efi machines the ACPI table
is not in the probed range.  This change grabs the address out of /sys
and specifies it explicitly on the kexec cmdline.

Fixes: https://github.com/coreos/bugs/issues/167
Merge pull request #38 from vcaputo/acpi_rsdp

Explicitly supply the ACPI RSDP address to kexec on efi systems