kernel-fitimage: unbreak UBOOT_ENTRYSYMBOL support

kernel-fitimage: unbreak UBOOT_ENTRYSYMBOL support

- vmlinux is located in ${B}, not ${S}.
- parsing of nm output got broken completely in commit
  b406a89935f148779569fa3770776e009dd51f13 ("kernel-fitimage: add
  initramfs support"), commit ec755d2524fcbd9dfded23a576f25c990d405a6c
  in yocto

While at it, make awk exit on match to save a few CPU cycles.

(From OE-Core rev: 9d2ec9c046c4a9c6a842d28133d40639f5a65297)

Signed-off-by: André Draszik <adraszik@tycoint.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>