.editorconfig: Add indent style & size of 2 spaces for shell

This adds a default style for shell scripts. fmtsh now looks for this
when reformatting shell scripts.

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: I348f23badf302a48c851231a08c1ce4be94738a4
Reviewed-on: https://review.coreboot.org/c/coreboot/+/78830
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

.editorconfig

@@ -9,3 +9,7 @@ charset = utf-8
 insert_final_newline = true
 end_of_line = lf
 trim_trailing_whitespace = true
+
+[*.sh]
+indent_style = space
+indent_size = 2