nrf24.h: Sync with optiboot version.
Hack to avoid sequences of three Tx ops followed by Rx.

This seems to break the last Rx for whatever reason.  I still need to
test this with more MCUs and more nRFs, but it looks like sending exactly
three Tx packets in a row cause the subsequent Rx packet to not be
reported on the SPI bus.  If there are fewer or more Tx operations,
everything is fine, only 3 TXs in a row cause this problem regardless of
the timing and the sizes of the packets.  So we try to count subsequent
Tx operations and split the 3rd one in a row into two packets.

Oh, well.
Add example jump-to-bootloader code.
Send max packet size in the hello packet.

Lower max packet payload may help reduce retransmit rate in low-RSSI
environments.
Software rexmit + SEQN mechanisms in addition to nRF24's builtin one.
nrf24.h: Sync with optiboot version.

Commits:
de19f5de3ef0d46060fbbb7e8968abff87c5e0dc
b07e9ad5f785e940af49c4b8f3ff4e3fb4626ae9
99cbdcfc74142d2dcf73437e106714f72e178d07
6d9baed264c8c7d990c74c6c0cc21101f7a18c20
158117d9f17809cd63894a572bde5c3e8e9dab93