efi_loader: switch to non-secure mode later

Some ARMv7 boards using PSCI require to be in secure-mode when booted via
'bootz' or 'bootm'. During distro-boot 'bootefi bootmgr' is called to check
if booting via UEFI is possible.

With the change we change the switch from secure mode to non-secure mode is
moved from the UEFI subsystem setup to just before calling StartImage().

Cc: Jernej Škrabec <jernej.skrabec@gmail.com>
Reported by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>