[efi] Maximise image base address

[efi] Maximise image base address

iPXE images are linked with a starting virtual address of zero.  Other
images (such as wimboot) may use a non-zero starting virtual address.

There is no direct equivalent of the PE ImageBase address field within
ELF object files.  Choose to use the highest possible address that
accommodates all sections and the PE header itself, since this will
minimise the memory allocated to hold the loaded image.

Signed-off-by: Michael Brown <mcb30@ipxe.org>