rpi-config: support more commonly used config variables and update documentation

Added support for and documented the following new extra build options:

* gpu_freq overclocking option
* disable boot rainbow splash screen
* configure the boot delay after GPU firmware loading
* configure common HDMI and composite video options

Signed-off-by: Hugo Hromic <hhromic@gmail.com>

1 files changed, 45 insertions, 3 deletions

diff --git a/docs/extra-build-config.md b/docs/extra-build-config.md
index d162abc..35abbdf 100644
--- a/docs/extra-build-config.md
+++ b/docs/extra-build-config.md
@@ -32,6 +32,8 @@ Accommodate the values above to your own needs (ex: ext3 / ext4).
 * `GPU_MEM_1024`: GPU memory in megabyte for the 1024MB Raspberry Pi. Ignored by
   the 256MB/512MB RP. Overrides gpu_mem. Max 944. Default not set.
 
+See: <https://www.raspberrypi.org/documentation/configuration/config-txt/memory.md>
+
 ## Add purchased license codecs
 
 To add you own licenses use variables `KEY_DECODE_MPG2` and `KEY_DECODE_WVC1` in
@@ -44,6 +46,7 @@ You can supply more licenses separated by comma. Example:
 
     KEY_DECODE_WVC1 = "0x12345678,0xabcdabcd,0x87654321"
 
+See: <https://www.raspberrypi.org/documentation/configuration/config-txt/codeclicence.md>
 
 ## Disable overscan
 
@@ -53,18 +56,57 @@ local.conf:
 
     DISABLE_OVERSCAN = "1"
 
+## Disable splash screen
+
+By default a rainbow splash screen is shown after the GPU firmware is loaded.
+To disable this set this variable in local.conf:
+
+    DISABLE_SPLASH = "1"
+
+## Boot delay
+
+The Raspberry Pi waits a number of seconds after loading the GPU firmware and
+before loading the kernel. By default it is one second. This is useful if your
+SD card needs a while to get ready before Linux is able to boot from it.
+To remove (or adjust) this delay set these variables in local.conf:
+
+    BOOT_DELAY = "0"
+    BOOT_DELAY_MS = "0"
+
 ## Set overclocking options
 
-The Raspberry PI can be overclocked. As of now overclocking up to the "Turbo
+The Raspberry Pi can be overclocked. As of now overclocking up to the "Turbo
 Mode" is officially supported by the raspbery and does not void warranty. Check
-the config.txt for a detailed description of options and modes. Example turbo
-mode:
+the config.txt for a detailed description of options and modes. The following
+variables are supported in local.conf: `ARM_FREQ`, `GPU_FREQ`, `CORE_FREQ`,
+`SDRAM_FREQ` and `OVER_VOLTAGE`.
+
+Example official settings for Turbo Mode in Raspberry Pi 2:
 
     ARM_FREQ = "1000"
     CORE_FREQ = "500"
     SDRAM_FREQ = "500"
     OVER_VOLTAGE = "6"
 
+See: <https://www.raspberrypi.org/documentation/configuration/config-txt/overclocking.md>
+
+## HDMI and composite video options
+
+The Raspberry Pi can output video over HDMI or SDTV composite (the RCA connector).
+By default the video mode for these is autodetected on boot: the HDMI mode is
+selected according to the connected monitor's EDID information and the composite
+mode is defaulted to NTSC using a 4:3 aspect ratio. Check the config.txt for a
+detailed description of options and modes. The following variables are supported in
+local.conf: `HDMI_FORCE_HOTPLUG`, `HDMI_DRIVE`, `HDMI_GROUP`, `HDMI_MODE`,
+`CONFIG_HDMI_BOOST`, `SDTV_MODE`, `SDTV_ASPECT` and `DISPLAY_ROTATE`.
+
+Example to force HDMI output to 720p in CEA mode:
+
+    HDMI_GROUP = "1"
+    HDMI_MODE = "4"
+
+See: <https://www.raspberrypi.org/documentation/configuration/config-txt/video.md>
+
 ## Video camera support with V4L2 drivers
 
 Set this variable to enable support for the video camera (Linux 3.12.4+