luci-app-attendedsysupgrade: use rpc-sys pkglist

luci-app-attendedsysupgrade: use rpc-sys pkglist

The bash package rpcd-mod-packagelist is no longer required as it has
been ported to rpc-sys by @dangowrt. The luci app now uses that feature.

The packagelist.acls is no longer required and has been removed.

luci-app-attendedsysupgrade: follow new server api

a new api uses more status codes and headers instead of genric 400 code
with json payload.

show error message when ubus call fail

fix typo

more info here
https://github.com/aparcar/attendedsysupgrade-server#response-status-codes

luci-app-attendedsysupgrade: refactoring

luci-app-attendedsysupgrade: use checksum

now compares cgi-io md5 checksum with md5 of server
as well checks if enough free memory is available to store image

Added needed ACL to let Luci request "system info"

rename "image" to "firmware" in output strings. This was requested as
"unification"

rpcd-mod-packelist used to return a variable called "packagelist", the
new "rpc-sys packagelist" returns a variable called "packages". This has
been adapted.

Signed-off-by: Paul Spooren <paul@spooren.de>