Type hint TROUT_KERNEL_IMAGE as a string

Since it's set to the value of a $(wildcard),
by default mk2rbc assumes it's a list. In a different
makefile, it's then assumed to be a string again,
and the list gets converted to a string, introducing
erroneous brackets and quotes around it's value.

Bug: 224601891
Test: rbc dashboard script
Change-Id: I6cd311c59342e342fc6f361a47573ccbd041ef3e
1 file changed