ARMv4 does not support the bkpt mnemonic. Rather than trying to work
around this, just disable this feature for ARMv4. On Tegra the PBL
runs on the ARM720T co-processor, so we also need to disable the option
for this SoC architecture.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>