package/brotli: properly pass TARGET_CFLAGS in Microblaze workaround

package/brotli: properly pass TARGET_CFLAGS in Microblaze workaround

In commit 39dde1ffb6a7111c2fa2b340c940e4d3e685aa29 ("package/brotli:
work around gcc bug 68485"), a mistake was made: when
BR2_TOOLCHAIN_HAS_GCC_BUG_68485=y, we're only passing -O0 as CFLAGS,
completely ignoring TARGET_CFLAGS, which is not good.

This commit fixes that by introducing a BROTLI_CFLAGS variable,
initially defined to TARGET_CFLAGS, and to which we add -O0 when
BR2_TOOLCHAIN_HAS_GCC_BUG_68485=y.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
[Thomas: rework commit log]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>