installkernel: remove bashisms

[Proto1337]

Convert the installkernel script to POSIX sh by removing bash-specific syntax
that caused failures on systems where /bin/sh is not bash (e.g. dash):

• Replacing [[…]] with POSIX […] syntax (SC3010)
• Using = instead of == for string comparisons (SC3014)
• Replacing $EUID with $(id -u) (SC3028)
• Replacing echo -e and "\e" escapes with printf and "\033" (SC3037)
• Quoting variables to prevent globbing and word splitting (SC2086)

After this change, the script runs correctly with /bin/sh symlinked to dash.
It also passes ShellCheck with no POSIX warnings.

[Foxboron]

Is there a reason this can't just be bash?

[Proto1337]

There isn't a single reason.
I saw that the kernel-install script was POSIX-compliant so I decided to do the same here.

Honestly changing the shebang to #!/usr/bin/env bash would already do the trick. The installkernel scripts in Gentoo also use #!/usr/bin/env bash as shebang.

[Foxboron]

ACK. I'll merge this. It seems like this is intended for Debian but the conventions are taken from Gentoo. I suspect it makes more sense to have this POSIX compliant.

[Foxboron]

Thanks!