dropbear: honour CFLAGS while building bundled libtomcrypt/libtommath

Felix Fietkau pointed out that bundled libtomcrypt/libtommath do funny stuff with CFLAGS.
fix this with checking environment variable OPENWRT_BUILD in both libs.
change in dropbear binary size is drastical: 221621 -> 164277.

Signed-off-by: Konstantin Demin <rockdrilla@gmail.com>

package/network/services/dropbear/patches/901-bundled-libs-cflags.patch

@@ -0,0 +1,48 @@
+--- a/libtomcrypt/makefile_include.mk
++++ b/libtomcrypt/makefile_include.mk
+@@ -75,6 +75,13 @@ endif
+ 
+ LTC_CFLAGS += -Wno-type-limits
+ 
++ifdef OPENWRT_BUILD
++  ifeq (-Os,$(filter -Os,$(CFLAGS)))
++    LTC_CFLAGS += -DLTC_SMALL_CODE
++  endif
++else
++  ### ! OPENWRT_BUILD
++
+ ifdef LTC_DEBUG
+ $(info Debug build)
+ # compile for DEBUGGING (required for ccmalloc checking!!!)
+@@ -102,6 +109,9 @@ endif
+ endif # COMPILE_SMALL
+ endif # COMPILE_DEBUG
+ 
++  ### ! OPENWRT_BUILD
++endif
++
+ 
+ ifneq ($(findstring clang,$(CC)),)
+ LTC_CFLAGS += -Wno-typedef-redefinition -Wno-tautological-compare -Wno-builtin-requires-header -Wno-missing-field-initializers
+--- a/libtommath/makefile_include.mk
++++ b/libtommath/makefile_include.mk
+@@ -37,6 +37,9 @@ CFLAGS += -Wsystem-headers -Wdeclaration
+ CFLAGS += -Wstrict-prototypes -Wpointer-arith
+ endif
+ 
++ifndef OPENWRT_BUILD
++  ### ! OPENWRT_BUILD
++
+ ifdef COMPILE_DEBUG
+ #debug
+ CFLAGS += -g3
+@@ -58,6 +61,9 @@ endif
+ endif # COMPILE_SIZE
+ endif # COMPILE_DEBUG
+ 
++  ### ! OPENWRT_BUILD
++endif
++
+ ifneq ($(findstring clang,$(CC)),)
+ CFLAGS += -Wno-typedef-redefinition -Wno-tautological-compare -Wno-builtin-requires-header
+ endif