image: Fix FIT image loadable section custom processing

image: Fix FIT image loadable section custom processing

The original commit always generated linker list entries with the same
entry variable name, because _function passed to ll_entry_declare() is
not a variable and therefore was interpreted as fixed string. Change it
to _type which is a variable and which does allow generation of multiple
unique linker list entries, one for each U_BOOT_FIT_LOADABLE_HANDLER().

Fixes: d7be50921ed3 ("image: Add FIT image loadable section custom processing")
Signed-off-by: Marek Vasut <marek.vasut@mailbox.org>