xbps-src: set XBPS_CHROOT_CMD=uchroot with -t; improve desc for -a.

xbps-src

@@ -127,8 +127,8 @@ zap
 
 Options:
 
--a  <profile>
-    Cross compile packages for this profile. Supported targets:
+-a  <target>
+    Cross compile packages for this target machine. Supported targets:
 
 $(print_cross_targets)
 
@@ -182,6 +182,7 @@ $(print_cross_targets)
 -t  Create a temporary masterdir to not pollute the current one. Note that
     the existing masterdir must be fully populated with binary-bootstrap first.
     Once the target has finished, this temporary masterdir will be removed.
+    This flag requires xbps-uchroot(1).
 
 _EOF
 }
@@ -578,10 +579,7 @@ done
 # A temporary masterdir requires xbps-uchroot(8) and -O to use overlayfs
 # on tmpfs (available with xbps-0.45).
 if [ -z "$IN_CHROOT" -a -n "$XBPS_TEMP_MASTERDIR" ]; then
-    if [ "$XBPS_CHROOT_CMD" != "uchroot" ]; then
-        echo "ERROR: -t requires XBPS_CHROOT_CMD=uchroot, exiting..."
-        exit 1
-    fi
+    export XBPS_CHROOT_CMD="uchroot"
     export XBPS_CHROOT_CMD_ARGS+=" -O"
 fi
 #