riscv: increase the environment size for the qemu-riscv platform to 128kB

The existing default size of 4kB is too small as the default environment
has already nearly that size and defining a single additional environment
variable can exceed the available space.

Signed-off-by: Karsten Merker <merker@debian.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>