Fixed quote errors in echo expressions

Fixed quote errors in echo expressions

Enclosed the echo expressions in double quotes (which previously had single quotes) for the following pages to prevent build errors:
1. Customizing Your Build
2. Building the AGL Image
  a. Building for x86 (Emulation and Hardware)
  b. Building for Raspberry Pi 4
  c. Building for Supported Renesas Boards

Signed-off-by: amanarora9848 <aman.arora9848@gmail.com>
Change-Id: I18543d001e5da6fbdcf11d72236122c7e6ffdac1
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/27575
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>