alternatives: initial feature

This feature allows you to set alternatives [1].
For example, like this:

   @$(call add,ALTERNATIVES,/usr/bin/xvt:/usr/bin/xterm)

Also, in the config itself, targets for setting alternatives may already
be predefined. For example, use/alternatives/xvt/% allows you to specify
arbitrary alternatives for xvt:
use/alternatives/xvt/xterm, use/alternatives/xvt/mate-terminal, etc.
However, an alternative must be available.

1. https://www.altlinux.org/Alternatives

doc/mkimage-profiles.asciidoc

@@ -54,6 +54,8 @@ include::../features.in/README[]
 
 include::../features.in/00example/README[]
 
+include::../features.in/alternatives/README[]
+
 include::../features.in/apt-conf/README[]
 
 include::../features.in/armh-mcom02/README[]

features.in/alternatives/README

@@ -0,0 +1,13 @@
+=== features.in/alternatives ===
+
+Данная фича позволяет задавать альтернативы [1].
+Например, так:
+
+  @$(call add,ALTERNATIVES,/usr/bin/xvt:/usr/bin/xterm)
+
+Также в самом конфиге могут быть уже преопределены цели для установки альтернатив.
+Например, use/alternatives/xvt/% позволяет задавать произвольные альтернативы для xvt:
+use/alternatives/xvt/xterm, use/alternatives/xvt/mate-terminal и т.д.
+При этом альтернатива должна быть доступна.
+
+1. https://www.altlinux.org/Alternatives

features.in/alternatives/config.mk

@@ -0,0 +1,6 @@
+use/alternatives:
+	@$(call add_feature)
+	@$(call xport,ALTERNATIVES)
+
+use/alternatives/xvt/%: use/alternatives
+	@$(call add,ALTERNATIVES,/usr/bin/xvt:/usr/bin/$*)

features.in/alternatives/rootfs/image-scripts.d/99-alternatives

@@ -0,0 +1,10 @@
+#!/bin/sh -efu
+
+[ -n "$GLOBAL_ALTERNATIVES" ] || exit 0
+for alternative in $GLOBAL_ALTERNATIVES; do
+	candidate="${alternative#*:}"
+	alternative="${alternative%:*}"
+	alternatives-manual "$alternative" "$candidate"
+done
+
+alternatives-update