Fix handling of gpu_mem to match raspi-config

[ghost]

rc_gui currently handles setting gpu_mem slightly differently to raspi-config:

• If there is no gpu_mem specified in config.txt, raspi-config will assume the default gpu_mem setting of 64MB
  (PR Fix default gpu_mem for boards with >=1GB RAM RPi-Distro/raspi-config#131 pending to change this to the actual firmware default of 64 or 76MB, which depends on memory size)
• If there is no gpu_mem specified in config.txt, rc_gui will call vcgencmd get_mem gpu and default to that

The rc_gui version of the algorithm suffers from a minor problem (apart from the fact it doesn't match raspi-config): if the user has previously booted with a gpu_mem setting in config.txt (for example 320MB), then removed that line (perhaps using a previous run of rc_gui), then run rc_gui, it will prompt the user with a "default" of e.g. 320MB, not what the firmware actually defaults to in the case of nothing being specified in config.txt, i.e. 64 or 76MB. This PR fixes up rc_gui so that it behaves the same as raspi-config with RPi-Distro/raspi-config#131 applied.

[ghost]

Also added some extras to the README to document some packages I needed to install before I could build it on my Pi 4B.