grub_efi_get_ram_base() looks for the lowest available RAM address by
traversing the memory map, comparing lowest address found so far.
Due to a brain glitch, that "so far" was initialized to GRUB_UINT_MAX -
completely preventing boot on systems without RAM below 4GB.
Change the initial value to GRUB_EFI_MAX_USABLE_ADDRESS, as originally
intended.
Reported-by: Steve McIntyre <93sam@debian.org> Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org> Tested-by: Steve McIntyre <93sam@debian.org> Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>