utils: Add 'invoke/quiet'.

* gnu/build/bootloader.scm (G_): Remove.
(open-pipe-with-stderr, invoke/quiet): Move to...
* guix/build/utils.scm: ... here.  Use 'let-values' instead of
'define-values' because Guile 2.0 (the bootstrap Guile) doesn't know
about 'define-values'.
* po/guix/POTFILES.in: Remove gnu/build/bootloader.scm, and add
guix/build/utils.scm.
* tests/build-utils.scm: Remove import of (gnu build bootloader).