apt: add nativesdk support

1. Add nativesdk support
- Apply the same patches of native
- Generate the same apt.conf.sample of native

2. Create user '_apt' to fix apt runtime warning
[snip]
$ apt update
...
W: No sandbox user '_apt' on the system, can not drop privileges
[snip]

RP: Add comment about need to user
RP: Add user to selftest static-passwd
(From OE-Core rev: be36b825ada573b2a7df9884eaab78664928594a)

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-selftest/files/static-group

@@ -19,3 +19,4 @@ tracing:x:519:
 pulse:x:520:
 bind:x:521:
 builder:x:522:
+_apt:x:523:

meta-selftest/files/static-passwd

@@ -15,3 +15,4 @@ sshd:x:516:516::/:/bin/nologin
 pulse:x:520:520::/:/bin/nologin
 bind:x:521:521::/:/bin/nologin
 builder:x:522:522::/:/bin/nologin
+_apt:x:523:523::/:/bin/nologin