fix Relative file path does not match actual file.
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.
add math library openlibm support (1)open menuconfig (2)close math.h:build setup->Customize Header Files->math.h (3) select openlibm:Library Rountines->Select math library->Math Library fram openlibm (4)build Signed-off-by: yanghuatao <yanghuatao@xiaomi.com>