efi_loader: bootefi hello should use loadaddr

Command 'bootefi hello' currently uses CONFIG_SYS_LOAD_ADDR
as loading address.

qemu machines have by default 128 MiB RAM.
CONFIG_SYS_LOAD_ADDR for x86 is 0x20000000 (512 MiB).
This causes 'bootefi hello' to fail.

We should use the environment variable loadaddr if available.
It defaults to 0x1000000 (16 MiB) on qemu_x86.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>