Revert "Parallelize depend file generation"
This reverts commit d5b6ec450f.
parallel depend ddc does not significantly speed up compilation,
intermediately generated .ddc files can cause problems if compilation is interrupted unexpectedly
Signed-off-by: xuxin19 <xuxin19@xiaomi.com>
Fix an issue that math library header files are not exported by make export.
Create symbolic links of libmcs, newlib and openlibm header to nuttx/include.