binman: Correct fmap output on x86

Normally x86 platforms use the end-at-4gb option. This currently produces
an FMAP with positions which have a large offset. The use of end-at-4gb is
a useful convenience within binman, but we don't really want to export
a map with these offsets.

Fix this by subtracting the 'skip at start' parameter.

Also put the code which convers names to fmap format, for clarity.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/binman/test/94_fmap_x86.dts

@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: GPL-2.0+
+/dts-v1/;
+
+/ {
+	#address-cells = <1>;
+	#size-cells = <1>;
+
+	binman {
+		end-at-4gb;
+		size = <0x100>;
+		pad-byte = <0x61>;
+		u-boot {
+		};
+		intel-mrc {
+		};
+		fmap {
+			offset = <0xffffff20>;
+		};
+	};
+};

tools/binman/test/95_fmap_x86_section.dts

@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: GPL-2.0+
+/dts-v1/;
+
+/ {
+	#address-cells = <1>;
+	#size-cells = <1>;
+
+	binman {
+		end-at-4gb;
+		size = <0x100>;
+		u-boot {
+		};
+		section {
+			pad-byte = <0x62>;
+			intel-mrc {
+			};
+			fmap {
+				offset = <0x20>;
+			};
+		};
+	};
+};