How to set physical button to run app in android custom rom

I've got new mobile from ebay, Blackview bv6000, which has 3 special physical buttons

1. Camera
2. Sos
3. Ptt

Then i need to assign those buttons to run my app and some functions

I got stock rom from Blackview forum, after i extract system.img file and found

\usr\keylayout\Generic.kl

which contains list of key function that i need

key 212 CAMERA

...

#[AGOLD][2015.04.08]Fengjun merge
key 249 FUNC1
key 250 FUNC2
#[AGOLD]End

The question is how can i assign that key to run my app or function in my app when I'm in home screen or in other app or even the screen is off