lz4: don't build library and tools in parallel

lz4: don't build library and tools in parallel

Since commit e93fec6673e (lz4: bump to version 1.8.1.2) we use two make
targets, 'lib' and 'lz4'. Both targets depend on the static library.
When these targets run in parallel the two make instances that build the
static library race against each other. Split these targets to ensure
build order.

Should fix:
http://autobuild.buildroot.net/results/a8d/a8d956ff420f6a265c5c00b33646dbbc24ce2d48/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>