vexpress64: fvp: Fix automatic boot

Commit 90f262a6 ("vexpress64: Clean up BASE_FVP boot configuration")
cleaned up the usage of default address variables, but missed to update
the address for the kernel in the FVP's bootcmd definition.

Change ${kernel_addr} to read ${kernel_addr_r} to bring back the
automated boot for the fastmodel.
Also use "setenv" instead of the potentially ambiguous "set" on the way.

Fixes: 90f262a6 ("vexpress64: Clean up BASE_FVP boot configuration")
Signed-off-by: Andre Przywara <andre.przywara@arm.com>