imagebuilder: clean KDIR_TMP before building
Building images usally stores them in KDIR_TMP and then copies them over
to BIN_DIR. This is fine as rebuilding of images overwrites existing
images. When using the EXTRA_IMAGE_NAME variable frequently this fills
up the ImageBuilder KDIR_TMP folder since every built image is stored
forever.
This commit clears the KDIR_TMP folder before building a new image.
Below an example how sysupgrade.openwrt.org filled up after the release
of 22.03.0 where every created image contains a hash of the package
selction in the filename:
aparcar@asu-01:~/asu/worker1/cache/22.03.0$ du -d 1 -h
400M ./kirkwood
260M ./gemini
2.0G ./ipq806x
1.7G ./ipq40xx
8.1G ./ramips
4.0K ./octeon
495M ./sunxi
728M ./lantiq
1.8G ./rockchip
3.7G ./mediatek
4.0K ./realtek
5.4G ./mvebu
8.9G ./ath79
3.0G ./bcm47xx
14G ./bcm27xx
11G ./x86
4.0K ./bcm63xx
312M ./mpc85xx
600M ./apm821xx
5.4G ./bcm53xx
66G .
Signed-off-by: Paul Spooren <mail@aparcar.org>