[rtl] add err_o signal to IMEM modules

[stnolting]

This PR adds the err_o signal to the IMEM module(s). This signal is set if the IMEM is implemented as true ROM and software attempts to perform a write access to it, which will create a "device error".

If there is a write access to a read-only IMEM the 📚 default runtime environment trap handler will output an error message (in this example: store 32-bit word to address 0x0000_0004):

<RTE> Store access fault [DEVICE_ERR] @ PC=0x0000016E, MTVAL=0x00000004 </RTE>

edit

This PR also add the err_o signal to the processor's BOOTROM.

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ℹ️ Before this PR write access to the IMEM also raised a bus exception by not asserting the IMEM's ack_o signal, which resulted in a "device timeout error":

<RTE> Store access fault [TIMEOUT_ERR] @ PC=0x0000016E, MTVAL=0x00000004 </RTE>

[stnolting]

⚠️ TODO: update the IMEM modules in neorv32-setups when this is merged