target: Add weak API to get HLOS subtype.
target: Add weak API to get HLOS subtype.

Add support to return the HLOS subtype of the device.

CRs-Fixed: 696087
Change-Id: I91a18b6e06260144a184bced40b59a1ce13220ab

Foxda-Tech/argo8-bootable-bootloader-lk,jsr-d10/android_bootable_bootloader_lk,Foxda-Tech/argo8-bootable-bootloader-lk,jsr-d10/android_bootable_bootloader_lk,jsr-d10/android_bootable_bootloader_lk,Foxda-Tech/argo8-bootable-bootloader-lk,jsr-d10/android_bootable_bootloader_lk,Foxda-Tech/argo8-bootable-bootloader-lk,Foxda-Tech/argo8-bootable-bootloader-lk,jsr-d10/android_bootable_bootloader_lk