arm: Use armv8_switch_to_el1 env to switch to EL1

Use the environment variable armv8_switch_to_el1 to determine whether
to switch to EL1 at runtime. This is an alternative to the
CONFIG_ARMV8_SWITCH_TO_EL1 compile-time option.

The environment variable will be ineffective if the ARMV8_MULTIENTRY
config is used.

This is required by the Armv8r64 architecture, which must be able to
boot at S-EL1 for Linux but may need to boot at other ELs for other
systems.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>