Replace all 43 POSIX `[ ]` tests with bash `[[ ]]` across five board-side
package hook functions (preinst, postinst_base, postinst_finish,
postinst_update_uboot_bootscript, get_bootscript_info).
Normalise `=` to `==` in the `"$1" == "upgrade"` comparison.
Collapse paired `[ A ] && [ B ]` into a single `[[ A && B ]]` where possible.
Variables that were previously unquoted inside `[ ]` (e.g. ${BOOTSCRIPT_DST},
${BOOTSCRIPT_BACKUP_VERSION}) are now properly quoted.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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