Fix the SHELL for CHROOT target for LFS

We use the filter builtin function of make to retrieve the right
shell to use during the CHROOT phase, which is the same as the
one in CHROOT1.
Also do not assume envars exist when removing it at the beginning
of the SUDO phase.

LFS/master.sh

@@ -532,7 +532,7 @@ mk_LUSER: mk_SETUP
 	@touch \$@
 
 mk_SUDO: mk_LUSER
-	@sudo rm envars
+	@sudo rm -f envars
 	@sudo make BREAKPOINT=\$(BREAKPOINT) SUDO
 	@touch \$@
 
@@ -603,7 +603,7 @@ EOF
 fi
 (
     cat << EOF
-CHROOT:       SHELL=/tools/bin/bash
+CHROOT:       SHELL=\$(filter %bash,\$(CHROOT1))
 CHROOT:       $chapter6
 BOOT:         $chapter78
 CUSTOM_TOOLS: $custom_list