oxide-barcode: allow shorter serial and part numbers

[cbiffle]

This retains the original maximum length for both fields, but permits parsed strings to contain shorter sections. Shorter sections are right-padded with NULs for consistency with IPCC.

Fixes #1893

[hawkw]

Handful of very unimportant nits, feel free to disregard me.

[citrus-it]

I flashed these changes to a rev B bench gimlet which has an unlocked VPD, then replaced the VPD contents with a v2 barcode containing short part and serial numbers:

% pfexec humility vpd -i 10 -r
[
    ("FRU0", [
        ("BARC", [
            "0XV2:9131234:000:EVT0006",
        ]),
        ("MAC0", [
            [
                168,
                64,
                37,
                1,
                1,
                3,
                8,
                0,
                8,
            ],
        ]),
    ]),

and booted. The board properties (and their length) printed as the kernel starts up look good:

Boot properties:
        0xb3e8 baseboard-revision = len=4 00000000
        0xb398 baseboard-model = len=8 9131234
        0xb348 baseboard-identifier = len=8 EVT0006

and sending an IPCC ident command from the running OS also shows the expected results:

# /usr/platform/oxide/bin/ipcc ident
Serial: 'EVT0006'
Model:  '9131234'
Rev:    0x0

The t6init service detected that the data encoded in the T6 SROM was no longer correct, and replaced it. That also worked correctly:

# t6mfgadm srom vpd show -d 1
OFFSET  FLAGS   ID              PN              SN              MAC
0x0400  CIPS-   T62100-KR       9131234         EVT0006         -
0x0800  CIPSM   T62100-KR       9131234         EVT0006         a8:40:25:1:1:13
0x0c00  CIPS-   T62100-KR       9131234         EVT0006         -
0x1000  CIPSM   T62100-KR       9131234         EVT0006         a8:40:25:1:1:13
0x1400  CIPS-   T62100-KR       9131234         EVT0006         -
0x1800  CIPSM   T62100-KR       9131234         EVT0006         a8:40:25:1:1:13
0x1c00  CIPS-   T62100-KR       9131234         EVT0006         -
0x2000  CIPSM   T62100-KR       9131234         EVT0006         a8:40:25:1:1:13
0x2400  CIPS-   T62100-KR       9131234         EVT0006         -
0x2800  CIPSM   T62100-KR       9131234         EVT0006         a8:40:25:1:1:13
0x2c00  CIPS-   T62100-KR       9131234         EVT0006         -
0x3000  CIPSM   T62100-KR       9131234         EVT0006         a8:40:25:1:1:13
0x3400  CIPS-   T62100-KR       9131234         EVT0006         -
0x3800  CIPSM   T62100-KR       9131234         EVT0006         a8:40:25:1:1:13
0x3c00  CIPS-   T62100-KR       9131234         EVT0006         -
0x4000  CIPSM   T62100-KR       9131234         EVT0006         a8:40:25:1:1:13