Minor touchup of "serial_hsit.svg"

Adjusted line connecting R232/USB with buildserver
Prep for 'HSIT_MASTER' build

Started preparing for our first hardware-software integration test
master application. 'make VARIANT=HSIT_MASTER all' currently
returns error and is therefore not yet included in
"build_all_variants.sh".

Also abstracted the list of valid build variants into the variable
'USER_INFO_VALID_VARIANTS'
Introduced make variable 'TEST_SRC_DIR'

For referring to the directory of the source code common for test
builds