spl: fit: add SPL_FIT_IMAGE_TINY config to reduce code-size

A minor code-size increase from the changes for tracking the os-type
of FIT images and from infrastructure for recording the loadables into
the the loaded FDT, broke the builds for sun50i and some OMAP2+ devices.

This change adds a new config option (enabled by default for
MACH_SUN50I, MACH_SUN50I_H5 and ARCH_OMAP2PLUS) that does skips these
processing steps (bringing code size down to below the limit
again). The os-type is not evaluated, but assumed to be IH_OS_UBOOT
(i.e. taking the code-paths intended for backward-compatibility).

Note that enabling this config option precludes any useful downstream
processing, such as utilising a special calling convention for ATF or
OPTEE, based on the os-type of the loadables.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>