[OoB_5.8]: export to make files does not build.

Steps:
1. mbed export -i make_gcc_arm -m K64F
2. Navigate to root of example.
3. make clean
4. make

Expected result:
Makefile builds binary of example.

Actual result:
Compilation fail.
```
C:\Ashok\mbed_CLI\OoB_5_8\TLS\mbed-os-example-tls>make
C:/Ashok/mbed_CLI/OoB_5_8/TLS/mbed-os-example-tls/makefile:805: warning: overriding commands for target `.s.o'
C:/Ashok/mbed_CLI/OoB_5_8/TLS/mbed-os-example-tls/makefile:797: warning: ignoring old commands for target `.s.o'
"Compile: main.c"
../pk_verify/main.c:23:28: fatal error: mbedtls/config.h: No such file or directory
#include "mbedtls/config.h"
^
compilation terminated.
make[1]: *** [../pk_verify/main.o] Error 1
make: *** [all] Error 2

C:\Ashok\mbed_CLI\OoB_5_8\TLS\mbed-os-example-tls>
```

Makefile attached (remove txt extension - only used for uploading to GitHub).

[Makefile.txt](https://github.com/ARMmbed/mbed-os-example-tls/files/1830427/Makefile.txt)

Reproduce the failure with `mbed export -i make_gcc_arm -m K64F`, then run `make clean` and `make` in the generated example root. Inspect the attached Makefile and the reported `mbedtls/config.h` include failure, along with the duplicate `.s.o` warnings. Done means the exported Makefile builds the example binary successfully.

Written by the indexing model from the issue text.