[efi] Use EFI-native mechanism for accessing SMBIOS table
EFI provides a copy of the SMBIOS table accessible via the EFI system
table, which we should use instead of manually scanning through the
F000:0000 segment.
A bug in read_smbios_string() was causing the starting offset of the
SMBIOS structure to be added twice, resulting in completely the wrong
strings being returned.
Bug identified by Martin Herweg <m.herweg@gmx.de>