flatcar-postinst: Handle airgapped self-hosted nebraska instances

[jepio]

This is a backport of the equivalent of flatcar/scripts#1580 to 3815, without switching to ue-rs.

Tested on the full-response from the airgapped environment (in commit message):

$ cat flatcar-postinst
set -euo pipefail
set -x
umask 0022
OEMID=vmware
sysext_download() {
    ....
}
sysext_download "oem-${OEMID}.gz" "./var/lib/update_engine/oem-${OEMID}.raw" test.xml

Output:

$ bash -x flatcar-postinst
+ set -euo pipefail
+ set -x
+ umask 0022
+ OEMID=vmware
+ sysext_download oem-vmware.gz ./var/lib/update_engine/oem-vmware.raw test2.xml
+ local name=oem-vmware.gz
+ local target=./var/lib/update_engine/oem-vmware.raw
+ local from=test2.xml
+ local base=
+ local entries=
+ local hash=
+ local size=
+ local url=
+ local ret
+ SUCCESS=false
+ set +e
+ set -e
+ '[' test2.xml = release-server ']'
+ '[' test2.xml = bincache-server ']'
++ grep -m 1 -o 'codebase="[^"]*"' test2.xml
++ cut -d '"' -f 2
+ base=https://nebraska-example.org/flatcar/
++ grep -m 1 -o '<package name="[^"]*oem-vmware.gz"[^>]*' test2.xml
+ entries='<package name="extrafile-amd64-3815.2.0-oem-vmware.gz" hash="llmt48kEE1pZxHbhE86zTczZMmY=" hash_sha256="7c4095f16579402d73ea42a63be63b2c92d98457303a080399d2a5bb33f46f88" size="1536465" required="false"/'
++ echo '<package name="extrafile-amd64-3815.2.0-oem-vmware.gz" hash="llmt48kEE1pZxHbhE86zTczZMmY=" hash_sha256="7c4095f16579402d73ea42a63be63b2c92d98457303a080399d2a5bb33f46f88" size="1536465" required="false"/'
++ grep -o 'name="[^"]*'
++ cut -d '"' -f 2
+ name=extrafile-amd64-3815.2.0-oem-vmware.gz
+ url=https://nebraska-example.org/flatcar/extrafile-amd64-3815.2.0-oem-vmware.gz
++ echo '<package name="extrafile-amd64-3815.2.0-oem-vmware.gz" hash="llmt48kEE1pZxHbhE86zTczZMmY=" hash_sha256="7c4095f16579402d73ea42a63be63b2c92d98457303a080399d2a5bb33f46f88" size="1536465" required="false"/'
++ grep -o 'size="[0-9]*'
++ cut -d '"' -f 2
+ size=1536465
++ echo '<package name="extrafile-amd64-3815.2.0-oem-vmware.gz" hash="llmt48kEE1pZxHbhE86zTczZMmY=" hash_sha256="7c4095f16579402d73ea42a63be63b2c92d98457303a080399d2a5bb33f46f88" size="1536465" required="false"/'
++ grep -o -P 'hash="[^"]*'
++ cut -d '"' -f 2
+ hash=llmt48kEE1pZxHbhE86zTczZMmY=
++ echo '<package name="extrafile-amd64-3815.2.0-oem-vmware.gz" hash="llmt48kEE1pZxHbhE86zTczZMmY=" hash_sha256="7c4095f16579402d73ea42a63be63b2c92d98457303a080399d2a5bb33f46f88" size="1536465" required="false"/'
++ grep -o -P 'hash_sha256="[^"]*'
++ cut -d '"' -f 2
+ hash_sha256=7c4095f16579402d73ea42a63be63b2c92d98457303a080399d2a5bb33f46f88
+ rm -f ./var/lib/update_engine/oem-vmware.raw.tmp
+ curl -fsSL --retry-delay 1 --retry 60 --retry-connrefused --retry-max-time 60 --connect-timeout 20 -o ./var/lib/update_engine/oem-vmware.raw.tmp https://nebraska-example.org/flatcar/extrafile-amd64-3815.2.0-oem-vmware.gz
curl: (6) Could not resolve host: nebraska-example.org
curl: (6) Could not resolve host: nebraska-example.org
....

[pothos]

Thanks