"The devices I am using is
1] 8051/ARM7 "

(1) First clarify whether u r using 8051 or ARM7 because these two are totally differet processor and having no architech ressemblance .
if u r using 8051 then u can for ISP (In Circuit Programmable) devices from ATMEL/Phillips like 89s8252 , 89c51RD2 for firmware upgradation.

"how this firmware upgrade will happen when my linker file defines the code address as read only"

You can download HEX file generated as Linker output to ISP devices using ISP hardware & software(avail in market and lots of information of same is aval forum past messages)

It is defined as read only because it is program code and no program code/instruction can write on this area. Code is read only after it is loaded into IC but ISP code inside above ICs can write your code on these memory location during programming mode .

ISP ICs are supplied with inbuit boot laoder.
You can go through Data Sheets/Application notes about these ICs on ATMEL/Phillips site