when declaring the target to be `nuttx`,create an empty allsyms source file for it;
when the target is declared as something else,the link behavior of the `nuttx` target is cloned
and added to actually generate the allsyms file.
finally use allsyms_nuttx to overwrite the already generated nuttx
and regenerate binary outputs
Signed-off-by: xuxin19 <xuxin19@xiaomi.com>
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