bootloader: De-monadify configuration file generators.

bootloader: De-monadify configuration file generators.

* gnu/bootloader/extlinux.scm: Remove unneeded imports.
(extlinux-configuration-file): Use 'computed-file' instead of
'gexp->derivation'.
* gnu/bootloader/grub.scm (svg->png): Likewise.
(grub-background-image, eye-candy): Adjust accordingly, return
non-monadically.
(grub-configuration-file): Likewise, and use 'computed-file' instead of
'gexp->derivation'.
* gnu/bootloader/u-boot.scm: Remove unneeded imports.
* gnu/system.scm: Add 'lower-object' call.