Build: GNUARM: fix objcopy command

Set and use ${CMAKE_OBJCOPY} variable for objcopy instead of using
${CROSS_COMPILE}-objcopy. This allows the flexibility of passing the
objcopy path while invoking cmake build command [1].

[1]: For example:
cmake -S . -B cmake_build -DTFM_PLATFORM=arm/rss/rdfremont
-DTFM_TOOLCHAIN_FILE=toolchain_GNUARM.cmake -DCMAKE_BUILD_TYPE=Debug
-DCMAKE_C_COMPILER=<full-path-to>/arm-none-eabi-gcc
-DCMAKE_OBJCOPY=<full-path-to>/arm-none-eabi-objcopy

Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: I7578c96fb1fcfbadf71bc005378590af025025b0
1 file changed