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>