Make QEMU version, arch and flags overridable via environment variables

[xuxiaowei-com-cn]

Allow customizing QEMUVERSION and QEMUFLAGS without modifying Makefiles, so that users can use different QEMU configurations for cross-arch builds.

What type of PR is this?

/kind cleanup

#4417

What this PR does / why we need it:

Makes QEMU version, flags, and ALL_ARCH configurable via environment variables in three image build Makefiles:

• Changes QEMUVERSION from = to ?= so it can be overridden
• Extracts hardcoded --reset -p yes flags into a new QEMUFLAGS variable with ?= assignment
• Replaces the inline flags in docker run with $(QEMUFLAGS)
• Changes ALL_ARCH from = to ?= so target architectures can be overridden

This allows downstream users and CI pipelines to customize QEMU behavior and target architectures without patching the Makefiles.

Files changed:

• images/build/debian-base/Makefile
• images/build/distroless-iptables/Makefile
• images/build/setcap/Makefile

Which issue(s) this PR fixes:

None

Special notes for your reviewer:

The changes are identical across all three files — extracting hardcoded values into conditionally-assignable variables (?=). The default behavior is unchanged.

Does this PR introduce a user-facing change?

NONE

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: xuxiaowei-com-cn
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[k8s-ci-robot]

Hi @xuxiaowei-com-cn. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

Regular contributors should join the org to skip this step.

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[xuxiaowei-com-cn]

• While building Kubernetes for Loongson, I found that some configurations cannot be controlled via parameters, which is why I created this issue.
• I hope the official team can optimize the configuration to facilitate rapid adaptation by downstream users.
• https://github.com/kubernetes-loong64/release-loong64/blob/loong64/v0.21.1/add-loong64-support.patch