app: aboot: allocate memory for cmdline_final instead of dst_buf
app: aboot: allocate memory for cmdline_final instead of dst_buf

dst_buf is a temprory variable, cmdline_final will be returned
from the func. Better for code understanding.

CRs-Fixed: 687723
Change-Id: I4a7edd368322721a03dcab637a79b2f5c0a47edd

efidroid/lk,Foxda-Tech/polaris-bootable-bootloader-lk,Foxda-Tech/polaris-bootable-bootloader-lk,Foxda-Tech/polaris-bootable-bootloader-lk,efidroid/lk,Foxda-Tech/polaris-bootable-bootloader-lk,Foxda-Tech/polaris-bootable-bootloader-lk,efidroid/lk,efidroid/lk,efidroid/lk,CanarySolutions/little-kernel,CanarySolutions/little-kernel,CanarySolutions/little-kernel,CanarySolutions/little-kernel,CanarySolutions/little-kernel