dev-arm: Don't unconditionally overwrite bootloader params
dev-arm: Don't unconditionally overwrite bootloader params

The bootloader arguments were previously defaulting to a predetermined
value even if initialized elsewhere in the platform config script.
This commit fixes this issue by not calling the default initialization
routine if the bootloader is already defined.

Change-Id: Id80af4762b52dc036da29430b2795bb30970a349
Signed-off-by: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/3967

rallylee/gem5,sobercoder/gem5,gem5/gem5,rallylee/gem5,gem5/gem5,rallylee/gem5,sobercoder/gem5,TUD-OS/gem5-dtu,sobercoder/gem5,TUD-OS/gem5-dtu,sobercoder/gem5,sobercoder/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,sobercoder/gem5,rallylee/gem5,sobercoder/gem5,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,rallylee/gem5,TUD-OS/gem5-dtu,rallylee/gem5,gem5/gem5,gem5/gem5,rallylee/gem5