u-boot.mk: override default PATH to avoid pick hostpkg python

u-boot.mk: override default PATH to avoid pick hostpkg python

hostpkg python from packages feed can be picked when do a incremental
build because hostpkg has higher priority in PATH. It may lead build
faliure as it's heavily trimmed (e.g. lacks necessary modules).

For uboot which uses binman and intree dtc, this is forced as hostpkg
python will never provide those modules by default.

Signed-off-by: Tianling Shen <cnsztl@immortalwrt.org>