ShakeWake: Fixed instant wake after sleep issue in certain positions

[ic-27]

Added lastX var to track the previous x acceleration for correct calculation of speed. Closes #927.

[FintasticMan]

This looks good to me, but seeing as @geekbozu wrote the shake wake algorithm, I'd like to hear what they have to say.

[github-actions[bot]]

Build size and comparison to main:	Section	Size	Difference
text	406636B	0B
data	940B	0B
bss	53576B	8B

[Riksu9000]

The last commit should be squashed with the first one, also the comment doesn't seem useful or accurate, so you may remove it in another commit.