* Use rp2040-hal in all example (possibly through their bsp)
Some of the examples were using the cortex_m_rt::entry method which
misses the device specific spinlock re-initialisation.
This commits makes the usage more consistent by using rp2040_hal exported
macro as the only `entry` method used across examples.
- Clippy warns about empty loops, https://github.com/rust-lang/rust-clippy/issues/6161
- wfi allows to CPU to save some power
WFI was avoided in examples for fear of ill interactions with debuggers.
However the rp2040 debug port does continue to work, as long as the
relevant clocks are not disabled in SLEEP_EN0/SLEEP_EN1. (By default,
all clocks stay enabled in sleep mode.)
This patch replaces several different workarounds with just calling wfi.
