mkbootimg: Specify custom mkbootimg

Specify custom mkbootimg to generate the boot image for LT26.

Change-Id: I622a44dd7feb734ba05ac5d198f0011d2e09a5a5
Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Merge pull request #9 from andersson/mkbootimg

mkbootimg: Specify custom mkbootimg