configs: j721e_evm_a72: Enhance bootcmd to start remoteprocs

The A72 U-boot can support early booting of any of the R5F or C66x
or C71x 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: Jean-Jacques Hiblot <jjhiblot@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>