param_t is only used as an offset and we have <1000 params, so an uint16_t
is enough.
This saves roughly 1KB of RAM. We only do that on NuttX because normal
integers have better performance in general.
Previously on amd64, this was even 64bits because it was an uintptr_t.
A large buffer on the heap was not deallocated when parameters were saved,
but there were no changes to the parameters. In that case
parameter_flashfs_write() was not called, which was previously responsible
for freeing the buffer.
This patch moves the responsibility of freeing the buffer to the calling
side, which already explicitly allocates the buffer.
