ARM: bootm: Allow booting in secure mode on hyp capable systems

Older Linux kernels will not properly boot in hyp mode, add support for a
bootm_boot_mode environment variable, which can be set to "sec" or "nonsec"
to force booting in secure or non-secure mode when build with non-sec support.

The default behavior can be selected through CONFIG_ARMV7_BOOT_SEC_DEFAULT,
when this is set booting in secure mode is the default. The default setting
for this Kconfig option is N, preserving the current behavior of booting in
non-secure mode by default when non-secure mode is supported.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>