add dynamic-loader example

[gianlu33]

Fixes #3

I made a very simple application that loads dynamically an SM at runtime, using sm_load().

The SM is implemented in sm1.c. This file is not included with the other source files when the application is built, but rather it is built separately and the binary file is automatically hardcoded as a byte array into sm1.h using a python script. Later on, this array is passed as input to the sm_load() function.

During the loading process, the SM needs information about some symbols (e.g., exit, __unprotected_entry), hence the static-symtab.c file is used to declare such symbols.

The logic of main.c is to be decided: @jovanbulck what should this example do/show other than loading dynamically an SM? Just temporarily, this logic checks if the SM's symbols are added correctly to the global symbol table (see sancus-tee/sancus-support#11)

[jovanbulck]

Thanks a lot for the PR @gianlu33 ! This really helps to understand and maintain the dynamic-loading functionality!

I added the example to the GitHub Actions CI as per ccd91cf so we can test and adjust before merging 👍

I'll start a code review and I might directly push small changes, or I might follow up with requested changes if it's not clear

[jovanbulck]

Left some comments, will see now if I can start addressing them partially already

[gianlu33]

I very much agree that it is better to compile the SM in the Makefile, thanks for making these changes :)

Let me know if you need something else from me!

[jovanbulck]

This is now merged, I think it's very nice and really helps showing a nice aspect about Sancus! Thanks @gianlu33 for the PR! 👍