make-4.4 changed the behaviour of .SECONDEXPANSION to only
expand rules that were explicitly used by the currently built target.
Because leveldata used .SECONDEXPANSION to override an implicit pattern
rule with another implicit pattern rule, the override wasn't considered
for expansion and never used. By replacing the implicit pattern override
with a static pattern override, i.e. by explicitly listing the dependencies,
the rule takes precedence over the implicit pattern rule ($(BUILD_DIR)/*.elf)
and gets expanded.
This patch fixes the build for make-4.4, and was also tested with make-4.3.