This patch fixed the error "bash: mksdimg.sh: No such file
or directory" when building cv18xx_aarch64.
The issue is introduced by commit "bsp: cvitek: removed
useless files after using rttpkgtool".
In addition, in order to unify the logic with riscv as much
as possible, the name of the "milkv-duo256m" directory under
cv18xx_aarch64 is uniformly changed to "duo256m".
This patch also improve the README, adding instructions to
install xz-utils.
Signed-off-by: Chen Wang <unicorn_wang@outlook.com>
