iSLER CH573 Fixes

[iracigt]

Update iSLER to fix TX and RX on the CH573.

The problem with the CH573 is the DMA can only access a portion of RAM. We thus need to ensure the buffers for TX and RX fall in this section. We use the .dma_safe linker section previously defined (see #733). This requires adding a section attribute on the buffers. Since the .dma_safe section is uninitialized, we also have to manually initialize these buffers at start up. This adds no extra overhead, because if the section were initialized this would be happening anyway in start.

Major changes:

• Buffers fed into iSLERTX need to be marked with ISLER_BUF_ATTR. This satisfies both section and alignment requirements.
• The section used for buffers can be forced by setting FUNCONF_ISLER_BUFFER_SECTION to the section name, e.g. ".bss"
• BREAKING: FUNCONF_ISLER_SECTION is replaced by FUNCONF_ISLER_FUNCTION_SECTION which takes just the section name instead of the full attribute.
• On the CH571/3 only, the names LLE_BUF and LLE_BUF2 are flipped. This makes LLE_BUF consistently the receive buffer.
• Some internal iSLER registers are renamed to reflect more reverse engineering info
• Updated examples with these changes

Tested on CH573 for both TX and RX using ble_beacon and iSLER examples. Also tested with a simple TX and RX demo on the CH592 to confirm nothing broke.

Seeking review from @biemster

[biemster]

This is great, thanks!

[cnlohr]

That's the endorsement I need, @biemster .

Thanks, @iracigt