Updated CMake for latest Pico-SDK, including Pico2 support (issue #18)

[sean-lawless]

... also includes hello_world fix (issue #17). Upgraded C++ compiler to 2017. NOTE: Four (4) tests are now skipped due to missing C++ linker dependencies - if merged please open an issue for these.

[lurch]

NOTE: Four (4) tests are now skipped due to missing C++ linker dependencies

As GitHub isn't doing a very good job of rendering the diff here, it might be worth mentioning on this PR which of the tests are now skipped?

[sean-lawless]

NOTE: Four (4) tests are now skipped due to missing C++ linker dependencies

As GitHub isn't doing a very good job of rendering the diff here, it might be worth mentioning on this PR which of the tests are now skipped?

Apologies, it looks like I introduced a line ending issue with the CMakeLists.txt (thank you Windows)...

The commented tests are:
#add_subdirectory("tests/flatbuffer_utils_test")
#add_subdirectory("tests/kernels_elementwise_test")
#add_subdirectory("tests/kernels_l2_pool_2d_test")
#add_subdirectory("tests/kernels_l2norm_test")

The errors are link related, For example:

[flatbuffer_utils_test]: undefined reference to `std::__throw_length_error(char const*)
...
[kernels_l2norm_test]: undefined reference to `sqrtf'

[sean-lawless]

I fixed the line endings so CMakeLists.txt shows changes.

[petewarden]

Apologies for the delay on this one, I was missing the notifications (fixed now).

I actually have a prerelease 2350 and was able to get TFLM compiling with minor changes. I will return to that now the SDK is final.

One wrinkle is that this repo is actually generated from the upstream Google TFLM repo and then patched with a script. That means if we want to continue to be able to merge upstream changes, we'll need to make sure the RP2xxx changes are part of the patch process, not direct modifications.

Anyway, sorry again for the lack of love, I should have time this week to focus on this update.

[Hedda]

@sean-lawless FYI, pete has now committed update from latest upstream SDK -> 82ee61d

[petewarden]

Ah, looks like this is a fix for a separate bug from the Pico2 support? I'll dig into this and see where the incorrect code is coming from. If it's upstream, I'll add a patch in the sync process, otherwise I'll pull this part of the PR in directly. Thanks for the update!