Create the cli.gitignore mechanism to pass extra default cli options

Document that X11 mouse is not moving.

Convert x='' to x= on all scripts, and also fix case ;; indentations.

Add dummy value to QEMU's -trace enable= to prevent warning.

Expand built-in package choice rationale.

cli.gitignore.example

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+arch=x86_64
+gem5=
+#gem5=-g
+printf '
+#BR2_TARGET_ROOTFS_EXT2_SIZE="1024M"
+' > br2.gitignore
+cli_build="-a ${arch} -b br2.gitignore -b br2_x11 ${gem5}"
+cli_run="-a ${arch} ${gem5}"
+cli_rungdb="-a ${arch} ${gem5}"
+cli_rungdbserver="-a ${arch} ${gem5}"
+cli_rungdb_user="-a ${arch} ${gem5}"
+cli_trace_boot="-a ${arch}"
+cli_trace2line="-a ${arch}"