refactor(fdt): introduce common fdt_node_is_enabled()

refactor(fdt): introduce common fdt_node_is_enabled()

There are several users in the tree which want to check whether a given
FDT node is enabled or not: the "status" property holds that
information. So far all those users provide private implementations,
some of them having issues.

Export a generic implementation of that function in fdt_wrappers.h, as
a "static inline" function to not increase code size.
Also replace the existing implementation in Arm's fconf code, which had
a tiny bug in needlessly using the property length:
"status = [6f 6b 61 79 20];" would pass the check, where it should not.
The proper solution is also simpler: status must be a string, and
strings must be NUL-terminated in a DT. strcmp() would terminate on the
first NUL in *either* of the two strings it compares, so it would never
walk beyond the property boundary in the DTB.

Change-Id: I9d89093432f127c09add6cf5c93a725bc534e5de
Signed-off-by: Andre Przywara <andre.przywara@arm.com>