build: increase MAX_CUSTOM_ROOTFS_SIZE_MB to a reasonable value

[efahl]

Current value of 100 is less than default when building OpenWrt (104). Increase it to 1024 to allow large root file systems on devices that can accommodate them.

https://github.com/openwrt/openwrt/blob/61e8728d86d7c11e1a5adab4bd37dae6b3b6cf2b/config/Config-images.in#L306

Closes: https://github.com/openwrt/asu/issues/826

[aparcar]

Did you test how long it takes to have a rootfs of one gigabyte?

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.61%. Comparing base (5e65dec) to head (fa09e1e). Report is 20 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #827 +/- ## ========================================== + Coverage 80.75% 84.61% +3.85% ========================================== Files 15 15 Lines 977 923 -54 ========================================== - Hits 789 781 -8 + Misses 188 142 -46 ```

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[efahl]

Did you test how long it takes to have a rootfs of one gigabyte?

Here's the results from doing image builder runs of x86/64 generic at three root fs sizes:

$ time make image PROFILE=generic ROOTFS_PARTSIZE=512

I also checked the resulting size of openwrt-x86-64-generic-ext4-combined-efi.img.gz for each build, as that seems to be the biggest of the images produced. Times are the worst case of three runs at each size, rounded up to nearest second (variation was always < 1s, so one run would have sufficed).

ROOTFS_PARTSIZE=	real	user	img size
104	26s	18s	12M
512	48s	25s	13M
1024	74s	33s	13M

(Those dds take forever, don't they?)

On the production ASU server, I measure the "104" builds to take about 90s, so my build platform is running about 3x faster than the server. Scaling that 74s * 3 = 225s = 3.75m, call it ~4 minutes...

[aparcar]

@dangowrt what do you think about this?

[efahl]

Just did a build using 256 MB, seems to work just fine. My time estimate was a bit low, 3.5 minutes for this one, but otherwise 👍🏼

Image: https://sysupgrade.openwrt.org/store/1c2a8746a0b600c998b9bb7b873ef34c/openwrt-f2f3d6a0adb2-x86-64-generic-squashfs-combined-efi.img.gz
Downloaded https://sysupgrade.openwrt.org/api/v1/build/1c2a8746a0b600c998b9bb7b873ef34c to /tmp/owut-build-status.json
Status: 200
Detail: done
Build completed in 210 seconds.
build_at       = 2024-05-12T11:49:39.000000Z
version_number = SNAPSHOT
version_code   = r26300-da0cd9d764
rootfs_size_mb = 256