ulib: Allow undefined symbols in the library

At present it isn't possible for ulib to call a function outside the
library, since it produces a link error.

Relax this contraint, so we can call a main() function provided by the
main program.

Signed-off-by: Simon Glass <simon.glass@canonical.com>

Makefile

@@ -1057,6 +1057,12 @@ endif
 
 LDFLAGS_u-boot += $(LDFLAGS_FINAL)
 
+# When building as a library, allow undefined symbols (e.g., main)
+# which will be resolved when linking the final static binary
+ifdef CONFIG_ULIB
+LDFLAGS_u-boot += --unresolved-symbols=ignore-all
+endif
+
 # Avoid 'Not enough room for program headers' error on binutils 2.28 onwards.
 LDFLAGS_u-boot += $(call ld-option, --no-dynamic-linker)