flashrom: fix static build

Use $(TARGET_MAKE_ENV) to set $PATH so as to make sure Buildroot pkg-config is
used. This corrects pkg-config output, and fixes static build.

Fixes:
http://autobuild.buildroot.net/results/b6e/b6e9f3cb7fa8019e631c14d9c832174125ec64ba/
http://autobuild.buildroot.net/results/30c/30c83f9c60ac5c7359ddabdc1f190ab12433ae92/
http://autobuild.buildroot.net/results/2a5/2a5da78dd5b2922796e3010e4a0477cb9f6355cb/



and more.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>