My thoughts as well
So what I intend to do - Raspberry (SPI) → SPI of Atmega32U4.
I will send a steady stream of hex data that will program into the Atmega.
Can I do it using the UART ? Say, using some ISP bootloader ? Something similar to the
LPC2124 or so ?
Can my Rx/Tx of the pi be linked to the Tx/Rx of the Arduino to program it ?
I don’t know much about Pi programming.
Details of how to program the Mega328 chip over SPI bus are in the datasheet.
You don’t need to use a hardware SPI interface on the transmitter, you can use general I/O pins+software (SPI is just data+clock signals, no exact timing requirements like RS232).