hw/mips/boston: Use bl_gen_kernel_jump to generate bootloaders

Replace embedded binary with generated code.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20201215064507.30148-2-jiaxun.yang@flygoat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
[PMD: Split original patch as one for each machine (here boston)]
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>