configs: j7200_evm_a72: Enhance bootcmd to start remoteprocs

The A72 U-boot can support early booting of any of the Main or MCU R5F
remote processors from U-boot prompt to achieve various system usecases
before booting the Linux kernel. Update the default BOOTCOMMAND to provide
an automatic and easier way to start various remote processors through
added environment variables.

Signed-off-by: Suman Anna <s-anna@ti.com>