Migrate to the new 'targets' field of bootloader-configuration.

The old 'target' field is deprecated; adjust the sources to use the new
'targets' one instead.

* doc/guix-cookbook.texi<target>: Replace by 'targets'.
* gnu/bootloader/grub.scm: Likewise.
* gnu/installer/parted.scm: Likewise.
* gnu/machine/digital-ocean.scm: Likewise.
* gnu/system/examples/asus-c201.tmpl: Likewise
* gnu/system/examples/bare-bones.tmpl: Likewise
* gnu/system/examples/bare-hurd.tmpl: Likewise
* gnu/system/examples/beaglebone-black.tmpl: Likewise
* gnu/system/examples/desktop.tmpl: Likewise
* gnu/system/examples/docker-image.tmpl: Likewise
* gnu/system/examples/lightweight-desktop.tmpl: Likewise
* gnu/system/examples/vm-image.tmpl: Likewise
* gnu/system/examples/yggdrasil.tmpl: Likewise
* gnu/system/hurd.scm: Likewise
* gnu/system/images/hurd.scm: Likewise
* gnu/system/images/novena.scm: Likewise
* gnu/system/images/pine64.scm: Likewise
* gnu/system/images/pinebook-pro.scm: Likewise
* gnu/system/images/rock64.scm: Likewise
* gnu/system/install.scm: Likewise
* gnu/system/vm.scm: Likewise
* gnu/tests.scm: Likewise
* gnu/tests/ganeti.scm: Likewise
* gnu/tests/install.scm: Likewise
* gnu/tests/nfs.scm: Likewise
* gnu/tests/telephony.scm: Likewise
* tests/boot-parameters.scm: Likewise
* tests/system.scm: Likewise

gnu/system/images/hurd.scm

@@ -42,7 +42,7 @@
     (inherit %hurd-default-operating-system)
     (bootloader (bootloader-configuration
                  (bootloader grub-minimal-bootloader)
-                 (target "/dev/sdX")))
+                 (targets '("/dev/sdX"))))
     (file-systems (cons (file-system
                           (device (file-system-label "my-root"))
                           (mount-point "/")

gnu/system/images/novena.scm

@@ -39,7 +39,7 @@
     (locale "en_US.utf8")
     (bootloader (bootloader-configuration
                  (bootloader u-boot-novena-bootloader)
-                 (target "/dev/vda")))
+                 (targets '("/dev/vda"))))
     (initrd-modules '("sdhci-esdhc-imx" "ahci_imx" "i2c-dev"))
     ;(kernel linux-libre-arm-generic)
     (kernel-arguments '("console=ttymxc1,115200"))

gnu/system/images/pine64.scm

@@ -38,7 +38,7 @@
     (locale "en_US.utf8")
     (bootloader (bootloader-configuration
                  (bootloader u-boot-pine64-lts-bootloader)
-                 (target "/dev/vda")))
+                 (targets '("/dev/vda"))))
     (initrd-modules '())
     (kernel linux-libre-arm64-generic)
     (file-systems (cons (file-system

gnu/system/images/pinebook-pro.scm

@@ -38,7 +38,7 @@
     (locale "en_US.utf8")
     (bootloader (bootloader-configuration
                  (bootloader u-boot-pinebook-pro-rk3399-bootloader)
-                 (target "/dev/vda")))
+                 (targets '("/dev/vda"))))
     (initrd-modules '())
     (kernel linux-libre-arm64-generic)
     (file-systems (cons (file-system

gnu/system/images/rock64.scm

@@ -39,7 +39,7 @@
     (locale "en_US.utf8")
     (bootloader (bootloader-configuration
                  (bootloader u-boot-rock64-rk3328-bootloader)
-                 (target "/dev/sda")))
+                 (targets '("/dev/sda"))))
     (initrd-modules '())
     (kernel linux-libre-arm64-generic)
     (file-systems (cons (file-system