tools: ynl: extend CFLAGS to keep options from environment

Package build environments like Fedora rpmbuild introduced hardening
options (e.g. -pie -Wl,-z,now) by passing a -spec option to CFLAGS
and LDFLAGS.

ynl Makefiles currently override CFLAGS but not LDFLAGS, which leads
to a mismatch and build failure:
CC sample devlink
/usr/bin/ld: devlink.o: relocation R_X86_64_32 against symbol `ynl_devlink_family' can not be used when making a PIE object; recompile with -fPIE
/usr/bin/ld: failed to set dynamic section sizes: bad value
collect2: error: ld returned 1 exit status

Extend CFLAGS to support hardening options set by build environment.

Signed-off-by: Jan Stancek <[e-mail]>
Acked-by: Jakub Kicinski <[e-mail]>
Reviewed-by: Donald Hunter <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/net/ynl/generated/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
 
 CC=gcc
-CFLAGS=-std=gnu11 -O2 -W -Wall -Wextra -Wno-unused-parameter -Wshadow \
+CFLAGS += -std=gnu11 -O2 -W -Wall -Wextra -Wno-unused-parameter -Wshadow \
 	-I../lib/ -idirafter $(UAPI_PATH)
 ifeq ("$(DEBUG)","1")
   CFLAGS += -g -fsanitize=address -fsanitize=leak -static-libasan

tools/net/ynl/lib/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
 
 CC=gcc
-CFLAGS=-std=gnu11 -O2 -W -Wall -Wextra -Wno-unused-parameter -Wshadow
+CFLAGS += -std=gnu11 -O2 -W -Wall -Wextra -Wno-unused-parameter -Wshadow
 ifeq ("$(DEBUG)","1")
   CFLAGS += -g -fsanitize=address -fsanitize=leak -static-libasan
 endif

tools/net/ynl/samples/Makefile

···
 include ../Makefile.deps
 
 CC=gcc
-CFLAGS=-std=gnu11 -O2 -W -Wall -Wextra -Wno-unused-parameter -Wshadow \
+CFLAGS += -std=gnu11 -O2 -W -Wall -Wextra -Wno-unused-parameter -Wshadow \
 	-I../lib/ -I../generated/ -idirafter $(UAPI_PATH)
 ifeq ("$(DEBUG)","1")
   CFLAGS += -g -fsanitize=address -fsanitize=leak -static-libasan