Version independent external apps (standalone apps) (#2145)

This pull requests adds a new type of external app to the firmware: The standalone app.

Pros:

Will work after an upgrade.
Size of image is only limited by shared heap size of M0 (application) (64kb total).
Cons:

No full access to all functions in the main firmware. One well defined (and versioned) API handles all communication.
The Pacman app was converted to be the first the the new kind.

firmware/application/external/external.ld

@@ -23,7 +23,6 @@ MEMORY
      * Also need to consider processor memory map - reading 0xADxxxxxx generates a fault which may be better than unexpected behavior.
      * External app address ranges below must match those in python file "external_app_info.py".
      */
-    ram_external_app_pacman     (rwx) : org = 0xADB00000, len = 32k
     ram_external_app_afsk_rx    (rwx) : org = 0xADB10000, len = 32k
     ram_external_app_calculator (rwx) : org = 0xADB20000, len = 32k
     ram_external_app_font_viewer(rwx) : org = 0xADB30000, len = 32k
@@ -47,12 +46,6 @@ MEMORY
 
 SECTIONS
 {
-    .external_app_pacman : ALIGN(4) SUBALIGN(4)
-    {
-        KEEP(*(.external_app.app_pacman.application_information));
-        *(*ui*external_app*pacman*);
-    } > ram_external_app_pacman
-
     .external_app_afsk_rx : ALIGN(4) SUBALIGN(4)
     {
         KEEP(*(.external_app.app_afsk_rx.application_information));