u-boot-tools: Use PATH_MAX for path length

Fixes:
   | uboot-mkimage -D "-I dts -O dtb -p 2000" -F -k "/buildarea1/test/wr_build/wrtestLTS_secureboot/test1-what/test2-what/test3-what/test4-what/test5-what/test6-what/test7-what/test8-what/test9-what/test10-what/test11-what/test12-what/layers/xilinx-zynqmp/zynqmp_keys/fitImage-rsa2048-keys" -K "u-boot.dtb" -r /buildarea1/test/wr_build/wrtestLTS_secureboot/test1-what/test2-what/test3-what/test4-what/test5-what/test6-what/test7-what/test8-what/test9-what/test10-what/test11-what/test12-what/build/tmp-glibc/work/xilinx_zynqmp-wrs-linux/u-boot-xlnx/1_v2023.01-xilinx-v2023.1+gitAUTOINC+40a08d69e7-r0/build/fitImage-linux
   | /buildarea1/test/wr_build/wrtestLTS_secureboot/test1-what/test2-what/test3-what/test4-what/test5-what/test6-what/test7-what/test8-what/test9-what/test10-what/test11-what/test12-what/build/tmp-glibc/work/xilinx_zynqmp-wrs-linux/u-boot-xlnx/1_v2023.01-xilinx-v2023.1+gitAUTOINC+40a08d69e7-r0/build/fitImage-linux: Image file name (uboot-mkimage) too long, can't create tmpfile.

Signed-off-by: Mingli Yu <mingli.yu@windriver.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>