configs/releng/syslinux/archiso_pxe.cfg: add "checksum" and "verify" boot options

An ISO's checksum and GPG validation primarily matters when PXE booting, so it would be appropriate to enable these options for releng.
They should protect against booting a corrupt image.

configs/releng/syslinux/archiso_pxe.cfg

@@ -11,7 +11,7 @@ ENDTEXT
 MENU LABEL Arch Linux install medium (x86_64, NBD)
 LINUX boot/x86_64/vmlinuz-linux
 INITRD boot/intel-ucode.img,boot/amd-ucode.img,boot/x86_64/archiso.img
-APPEND archisobasedir=%INSTALL_DIR% archisolabel=%ARCHISO_LABEL% archiso_nbd_srv=${pxeserver}
+APPEND archisobasedir=%INSTALL_DIR% archisolabel=%ARCHISO_LABEL% archiso_nbd_srv=${pxeserver} checksum verify
 SYSAPPEND 3
 
 LABEL arch64_nfs
@@ -22,7 +22,7 @@ ENDTEXT
 MENU LABEL Arch Linux install medium (x86_64, NFS)
 LINUX boot/x86_64/vmlinuz-linux
 INITRD boot/intel-ucode.img,boot/amd-ucode.img,boot/x86_64/archiso.img
-APPEND archisobasedir=%INSTALL_DIR% archiso_nfs_srv=${pxeserver}:/run/archiso/bootmnt
+APPEND archisobasedir=%INSTALL_DIR% archiso_nfs_srv=${pxeserver}:/run/archiso/bootmnt checksum verify
 SYSAPPEND 3
 
 LABEL arch64_http
@@ -33,7 +33,7 @@ ENDTEXT
 MENU LABEL Arch Linux install medium (x86_64, HTTP)
 LINUX boot/x86_64/vmlinuz-linux
 INITRD boot/intel-ucode.img,boot/amd-ucode.img,boot/x86_64/archiso.img
-APPEND archisobasedir=%INSTALL_DIR% archiso_http_srv=http://${pxeserver}/
+APPEND archisobasedir=%INSTALL_DIR% archiso_http_srv=http://${pxeserver}/ checksum verify
 SYSAPPEND 3
 
 INCLUDE boot/syslinux/archiso_tail.cfg