aarch64: install simple vectors and get the cmdline

get the command line and elf header start, then try to
stay clear of apparent random limitations during boot
early stage with the model, and set up vectors as soon
as possible, to enable some minimal post-mortem info.

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
aarch64: downcase regs in boot.S, do not set sctlr_el1 yet

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>