Switch to using uncompressed kernels by default and avoid vmlinuz/initrd.img copies

We want to avoid this since it makes roll backs to older kernel versions harder and wastes space (not so much due to the kernel but more due to the initramfs and the backup files).

tests performed:

• checked that boot.scr has the versioned entries
• checked that installing with linux-image-librem5_5.16.7pureos1 installed fails
• checked that installing with linux-image-librem5_5.16.7pureos2 installed works
• checked that /boot/{vmlinz,initrd.img}{.bak} get no longer generated (speeding up flash-kernel too)
• tests that switching kernel versions with --force works as long as the vmlinuz-<ver> is uncompressed.

Closes: linux-next#396

This reverts commit 58b67e3f.